The 11eyes OVA becomes one at the end. Although they are wearing bathing suits and its mixed bathing.
801 T.T.S. Airbats: The fourth episode has the gang stopover at a local hotspring that turns out to be haunted. Or so they're lead to believe.
Not in the series proper, but the OVA episode "Vacation" for Accel World centered around Haruyuki's friends taking him to a hot spring to help him lose weight for an upcoming physical exam. The place serves for a bit of soul searching for both him and his girlfriend Kuroyukihime. In playing with the series's rules, there also exists a virtual counterpart to the hot springs in the Accelerated World, which the characters take a dip in as their avatars.
In the Ah! My Goddess manga, Keiichi wins a trip to a Hot Springs Resort. Due to a series of mishaps with Chihiro and their ride, Belldandy conjures up an impromptu Hot Springs near where they breakdown to at least salvage the situation.
Aikatsu!: Subverted in episode 24 of anime season 1. The audition sequence ends before the opening, the main trio is left to spend a short break opting to visit a certain idols-approved hot springs inn. They end up captivated by scenery on the final bus stretch, get off, have a picnic and a nap, with too little time left to perhaps provide the series with that kind of fanservice.
Ai Kora: Played for laughs and lampshaded in the same chapter, when Hachibe invites all the girls along with him to an onsen he "won" tickets for (he really worked at a bunch of several part time jobs to earn the money to buy them, unsurprisingly.) simply so he can talk to Sakurako and figure out his feelings for her and vice versa, and also to peek at the girls of course.
Ai Yori Aoshi did this twice. In one of them, a little boy is frightened by Kaoru's terribly scarred back. This actually provides a Fridge Brilliance insight into his state of mind at this point. The fact that he didn't think of it shows that Kaoru's Dark and Troubled Past isn't so much on his mind anymore, thanks to his True Companions.
Having brought Sae to the hot springs for work training, after hearing her continuous yelps from the fish nipping her legs, Tachibana passes out from horniness.
And underclassman and swimming club member Ai confesses at a hidden hot spring while nude.
Anpanman has the character Onsen-kun, who actually makes hot springs. Every starring episode he gets will be a hot springs episode. The show is meant for a young audience, so no Fanservice is intended.
Happens in one episode of ARIA with everyone wearing Modesty Towel, but with a variation: most of the scene are set in the closed-air hot spring.
In the manga... how does North Italy find out that Japan's a lightweight in his pants? When they both take a bath in the same spring.
Also, Hungary and Taiwan bond over their love for hot springs via taking a trip to Lake Heviz, which is basically a huge Hungarian lake acting as such.
Black Clover has a four chapter/two episode mini-arc about this. Most of it is spent just getting to the hot spring, which is a reward for a training exercise that involves climbing a magically super-active volcano.Mereoleona apparently intended the mixed-gender group to all bathe together (to everyone else's discomfort) before Sol creates a wall to divide the pool in half. Also worth noting is that none of the guys are interested in peeking until Yami talks them into it, and seems more interested in watching them try than doing it himself. Yuno remains disinterested, though, while Asta refuses out of principle. The anime episode based on this chapter even ends with the words "So ends the obligatory hot springs episode".
The third OVA for Blue Seed Beyond was a long contrived Hot Springs episode where the team had to stay in the onsen itself or else a bomb would detonate and kill them all. Of note, the main plot was already resolved in the second OVA, so this was apparently nothing more than an excuse for having female characters wear swimsuits (with Kusanagi in a speedo as a bonus).
In Brave10, the Braves go to Yukimura's private onsen as a reward after Ishikawa Goemon's attack on Ueda in the manga. Since this fight was cut from the anime, it appears instead in the second half of episode six after Seikai's introduction.
Char's Deleted Affair (a Gundam Expanded Universe manga): An artificial hot spring is constructed on Axis and the three women who are kinda interested in Char wind up trying it out, including a younger Haman Karn.
Daily Life with Monster Girl has a hot springs chapter when the main characters go test out a new hot spring resort that's designed to cater to both humans and liminals before it opens (the house was undergoing some repairs at the time anyway). Due to how fan-service driven the series is to begin with, the chapter doesn't stand out in that regard.
Episode 6 of Date A Live is dedicated to everybody trying to get to the hot spring, only to face obstacles on the way. When they finally make it, Yoshino accidentally freezes it solid. Fortunately, Kannazuki, who had been ordered to dig a massive hole as punishment at the beginning of the episode, uncovers a geyser, and they convert that into an onsen.
In Detective School Q, two multiple murders happen in hot springs. Only the second one had fanservicy scenes: Kintarou first attempts to forcibly strip Ryuu off his modesty towel, then he and Kyuu try to peep on a Sexy Secretary who's bathing at the other side but end up seeing Megu nude instead. And the latter incident ends up being one of the clues to catch the Serial Killer... who happened to be said Sexy Secretary.
The Digimon series has invoked one nearly every season.
In Digimon Xros Wars, Akari, the lone girl in the party at the time, is alone on her side of the rocks. When she notices the villains of the week approaching, she peeks around the side to tell Taiki and Zenjirou. Zenjiro gets all freaked out and accuses her of peeping.
El Cazador de la Bruja has the main characters visit a hot spring in the middle of a Mexican desert. Lampshaded by having the characters actually say that it's common in Japan, and then by having one woman go in without a swimsuit on the grounds that that's how they do it in Japan.
One in Fairy Tail with Lucy and Erza bathing together during the Loke arc.
There is another one in chapter 261 with Lucy, Erza, Wendy, Levy and Juvia. Natsu, Gray, Jet and Droy try to do some Outdoor Bath Peeping. This was initially omitted from the TV anime but was instead made as the fourth OVA where the fanservice was increased.
There's another at the end of the Sun Village arc. Interestingly this one is unisex with Lucy and Wendy embarassed (Erza doesn't really seem to care) until they notice the guys aren't checking them out at all. They point out that thanks to their usual wacky antics they've accidentally seen them naked so many times the novelty is gone, and they proceed to enjoy their bath together without any incident.
In The Familiar of Zero, a hot spring erupts on Saito and Louise's land. Saito quickly converts it into a onsen and invites everybody to use it. Several characters had never heard of a hot spring, but agree to come when it is explained, and the girls like how it is good for the skin. They agree to have the boys and girls take turns, but the boys (except for Saito) predictably try to peep, only to be blasted away. Tiffania faints from the heat and has to be carried out. Later that night, Saito and Louise use the onsen together.
Fruits Basket has an episode where Kyo, Tohru, Momiji and Yuki visit the hot springs resort owned by the Sohma family. Not as much fanservice for guys, but a fair amount for the female audience.
Full Metal Panic? Fumoffu. Episode 9, alternatively titled "The Hot Springs Episode", is a direct parody of the hot springs cliché (among others), and features three male characters trying to infiltrate a women's bath, fully protected by a steel wall between the male and female sides to prevent peeking, sentry guns and mines.
There was also a bit of one in the original Full Metal Panic! in the bath on the TdD.
Episode 19 of The Galaxy Railways has the members of Sirius, Spica, and Vega platoons vacationing at a Hot Springs ouned by Captain Murase's parents.
Ginga Densetsu Weed: Tesshin and his pack lead Weed and the soldiers to hot springs to recover from their wounds and enjoy themselves. While there, Jerome (who went missing the episode before) is revealed to be recovering from a bullet wound, and Tesshin gives Weed a moving speech Gin told him to give to him. It's also where Weed learns to perfect the Zetzu Tenrou Battouga and uses it on a group of wild boars attacking the dogs.
Gin Tama has this on episodes 131 to 134 as Stand Arc where the Yorozuya and Otae-san took on a vacation and zany misadventure about spiri... stands.
Then on Episodes 219-B and 220, Gintoki, Shinpachi and the Shinsengumi are on this trope, now with Hedoro and his family. Hilarity ensues.
The one in Girls Bravo takes place in episode 11, when Yukinari, Kirie, and Koyomi take a trip in the countryside. Except Fukuyama and his sister, Lisa, decide to follow them. So their peaceful trip turns into a ping pong match between Kirie and Fukuyama to decide whether they got the boot, or if Kirie would have to use her naked body as a loofah to wash Fukuyama's back, if she lost.
Gravion, naturally, has a Hot Springs Resort episode, and naturally (being Gravion), it still ends with a Giant Robot battle.
One of the earlier episodes of Green Green has the school (which has just become co-ed) opening a hot spring, and focuses mainly on the boys trying to peek on the girls, ending with the boys all breaking the fence and spilling right on top of the nude girls (no towels, with the level of fanservice the anime exhibits), and later the main character searching underwater with one of his love interests for a key, to decide whether or not the guys get access to the springs. The springs appeared at times in later episodes.
Hayate the Combat Butler: Lampshaded and subverted in Ch. 117. After the characters spent about 5 chapters trying to get to a special hot springs, the first thing Hayate and Maria do is to relax... in massage chairs. Another character even exclaims, "Why hasn't the bath scene appeared yet?!"
The anime has at least two episodes centered around hot springs over its two seasons. The male main character isn't interested in peeking, unashamedly entering one of the springs when his (female) master has the effect of getting drunk. It seems the 'girls' are more likely to peek at each other in this story. And Ayumu is the only one who does that much.
Hell Girl has one where Hell hotline was instrumental in the building and destruction of a hot spring.
InuYasha has one every dozen or so episodes, particularly early on; the villagers find Kagome odd for her love of "purifying herself" in springs, but she eventually turns Sango on to hot spring bathing. This results in one of the series' Running Gags - Celibate Hero Inuyasha charging to the rescue after Kagome screams (sometimes even for him) only to end up an Accidental Pervert and subjected to a "sit" command. Once Chivalrous Pervert Miroku joins the band, this escalates to Inuyasha's efforts to deter his Girl Watching not only getting the monk clonked in the head by Sango's boomerang (even if she's not CARRYING it), but Kagome "sitting" Inuyasha just for being present.
Iono the Fanatics has the cast spending approximately half of the huge first chapter in a hot spring.
Second season of Junjou Romantica has Usagi and Misaki vacationing at a onsen, where they unfortunately meet Usagi' father Akihiko.
In Katekyō Hitman Reborn! they play the cheap version in when Dino's pet turtle destroys their bath tub. (and most of the house)
Kemono Friends has a fairly fanservice-free hot springs sequence in Episode 9 after Kaban and Serval manage to restore power to the mountain lodge. Amusingly, this bit reveals that the Friends are actually wearing clothing, something even they didn't realize until Kaban said that people usually undress before bathing ("You mean this stuff comes off?!").
Featured in a number of chapters in Kochikame when the main characters relax in an onsen. Featured in a number of Ryotsu's childhood flashbacks.
Koibana Onsen is about three sisters running an onsen. Twice, they take a vacation to visit an.... onsen!
Episode 2 of Last Period has the heroes take a quest at a hot spring, which has been losing business due to a Spiral blocking the water.
Love Hina is set in a girls' dormitory that used to be a hot springs resort; the girls living there use the spring, and the entire show can be considered a single continuous Hot Springs Episode (except, of course, for its several Beach Episodes).
Early on, Mahou Sensei Negima! involves quite a bit of action in the pool-sized bath, although not that kind of action. It dies down until a "Chapter 100% in the nude" appears as a break involving hot springs.
One early chapter earns points for confining a hot bath to a steel drum barely big enough for one person. There were two in it.
Majin Tantei Nougami Neuro has an onsen episode in which Yako does not even get to enjoy the hot springs because one of Neuro's devilish creatures was set loose on the water.
Maken-ki! has two, in chapters 4 and 33 respectively:
And while they're vacationing on Okino Island, the girls decide to bathe at an outdoor spring... without the guys. But the two of them escape and try to make in time to see some some eye candy. It then becomes a test of their determination, as they try to get past Yuuka'straps, to reach the spring before the girls are finished bathing.
The titular ship and setting of Martian Successor Nadesico had a simulated hot spring on board as a sauna area. Naturally, in one episode, local pervert Uribatake and his all-male engineering crew just happen to be searching for a Jovian infiltrator when most of the female cast is using the facility. The hot springs room gets checked out quite thoroughly.
Mazinger Z: In chapter 8 of the original manga, Kouji and his friends spending a while on a hot springs resort in a mountain. That episode was noteworthy because Mazinger-Z did not show up and it had a very funny Kouji/Sayaka moment in the hot springs (with a double Luminescent Blush included).
Miss Kobayashi's Dragon Maid has a Hot Springs Episode take up the second half of its OVA; while there's some fanservice, it's downplayed in favor of giving some character focus, something that's lampshaded by the episode's title "Valentines and Hot Springs! (Please Don't Get Your Hopes Up)". The relevant manga chapter shows a bit more, averting Nipple and Dimed at least for Tohru and Lucoa.
My Brother's Husband has Chapter 16, where Kana insists on visiting an onsen with her uncle Mike, and her father Yaichi has to invite her mother Natsuki along so that Kana has a female chaperone at the single-sex baths. What fanservice there is is low key and with the guys as the objects of attention.
My Monster Secret. Unusually for this trope, almost all the chapters involving it are set in the mens bath. There's still the obligatory fanservice, courtesy of Shiho showing up for the purposes of training up her "pervert power." This promptly leads to Rin also invading the men's bath to attempt to "seal" said pervert power.
Naruto has had a few hot springs episodes, mostly played up for comic value.
In one episode, Naruto and friends use the hot springs as an excuse to see Kakashi's real face, only to find that he bathes with his mask on.
In the same episode, Sakura, who is presumably naked, is seen kneeling in the adjacent bath, though they chose to give her a Modesty Towel instead of Censor Steam.
In another, the cast stops by a resort on the way back from a long plot arc. Lovable Sex Maniac Jiraiya immediately heads for the springs to take advantage of all the Fanservice. The females, in a moment of clarity decide to spend their time in the casino rather than the water. While the plot unfolds we are treated to repeated cutaways of Jiraiya sharing the water with monkeys, Sumo Wrestlers, and elderly old women. By the time the young women show up, Jiraiya has been in the hot water so long he's passed out from heat stroke.
The post-Time Skip series Shippuuden continues in the same vein. When the new team stays at the onsen, the bathing scenes are just... bathing scenes, though we do see Naruto briefly considering peeking on Sakura. Then he's told what happened when Jiraiya tried it on Tsunade... and Sakura's her apprentice, with temper and strength to match. He decides he likes his ribs and internal organs where they are.
In one of the post-episode vignettes, Naruto takes the Sand siblings on a tour of Konoha, including the hot springs which the Sand doesn't have. Sand ninja fan service.
During some Shippuden Filler we see hot spring action among Team 8 before the Time Skip. Comedy ensues when Shino's bugs also come out to enjoy the spring, freaking Kiba out and amusing Kurenai and Hinata from the other side of the divided, bug-free hotspring.
Most of the finales from the Onegai My Melody gradually turn into this, for some time.
One Piece: At the end of Arabasta arc, although the fanservice there is strictly for females. When Nami catches the guys peeking on her and Vivi, she gets rid of them by flashing them, causing them to pass out and fall back on their own side. She dubs this technique the "Happiness Punch" or "Paradise Punch", depending on the version.
There's a scene in a similar vein to a hot springs scene in the Amazon Lily arc where Luffy gets cleansed after being set on fire by the natives of the island. Despite this being Lady Land, only Marguerite, Luffy's eventual arc friend, is missing any clothing (Granted, the Amazons tend to wear little to begin with.), for no apparent reason other than law of fanservice and virtue of not being a Gonk.
Outlaw Star Episode 23, "Hot Springs Planet Tenrei" (as might be suspected from the title) dropped its Hot Springs Episode dead smack dab in the middle of the series' climactic plotarc. This episode was dropped from the dubbed airing in the U.S. simply because of how much nudity would have to be edited out of it. However, unlike typical fanservice episodes, there is a significant plot point in this one that may lead to confusion in following episodes, as Gene obtains four exceedingly rare caster shells; that's why he really went there in the first place: he even spent most of the episode ignoring the fanservice to the point that Jim started to get worried about him.
Patlabor had one sento episode. Which was a subversion in several ways. Firstly, it followed the male members into the bath, to locate a mad-bomber with "three moles in the left armpit". Fanservice was not a major point (though ye olde black dots got a workout), the guys never even tried to sneak a peek at the ladies' side. Ironic since the bomber was a woman.
In a later Patlabor OAV "Versus" , the characters are only seen coming back from the springs and the whole episode is about a contest/rivalry between Kanuka Clancy (back off the bus, on vacation) and her replacement Takeo Kumagami. Which starts with trivia questions, escalates through drunken tongue-twisters, and ends up with seeing who can throw Ohta out a window better. The only character we see in a spring is Captain Gotoh (and Ohta, but he's wearing a robe).
Episode 4 of the original OAV ("The Tragedy of L") features Captain Gotoh sending the SV2 pilots to a rural training camp, which includes a communal bath. (We get to see that Kanuka Clancy keeps a handgun with her even while she's in the bath.) During the bath scene, the water on the men's side suddenly turns red, scaring them, and from that point onward the episode becomes a ghost story. Turns out it was just a hoax plotted by the two captains, to teach the pilots a lesson about gun safety.
There's a hot springs scene in the first-season episode "Dig Those Diglett". It initially looks like Misty is topless in the spring, but when they rise after Pikachu points out some Diglett, it turns out she has a strapless top on her bikini. Another interesting thing is that they are not in a partitioned spring.
The anime had a hot springs episode during the Cinnibar Island arc, where Ash and Brock are on the boy's side and Misty - wrapped in a Modesty Towel is on the girl's side... until Togepi demolishes the wall between them by activating the secret door to Blaine's Gym.
In both, the Johto Saga and the Sinnoh Saga, there were two different episodes focusing on some Swinub finding hot springs. The Sinnoh one also introduces one of Dawn's childhood friends.
During the Lavaridge Town two-parter, Ash and his friends take a sand bath in the first episode after cleaning up the Gym's battlefield, while in the second episode, Team Rocket take a bath in some hot springs after blasting off...until they piss off three Ursaring and blast off the second time.
The third episode of the Unova Saga is one. Right after Ash received his Oshawott, he and Iris intend to do a sand bath, only for them to later find lout that the sand will be flooded by hot geyser. The sand bath house becomes a hot springs house in the end. And Ash meets the Sunglasses Sandile that would be caught by him 61 episodes later.
The manga had three incidents involving hot springs. One in the G/S/C arc was used by Sabrina and, later, Red. This was not due to fanservice; they were both frostbitten by Lorelei in the previous arc and needed to recover.
And then there's the Lavaridge Gym battle between Sapphire and Flannery — yes, a Gym battle in a hot spring. Sapphire wanted to do it right then and there, however; Flannery and Mack used their strongest fire powers to slow down Mt. Chimney's complete death after Team Aqua killed the volcano, and it was now or never for the hot springs.
Ranma ½ has both variations; the bathhouse more often than the springs (the bath seemed to break every other week and this was sometimes just used to start the story). One time the hot spring is large and circular, with a rock outcropping providing a wall through the middle, separating it into men's and women's sides. The men try to peek around one end, and the women try to peek around the other end at the same time. They don't see anything, so they dismiss the sounds they heard as delusions.
Somewhat subverted on Read or Die: The TV. They all go to the hot springs, but none of the main characters remove so much as their jackets. The one 'guest star' who does take off her clothes ends up brutally murdered. In the spring. And then her bodyguard activates an ancient device that destroys the springs entirely in a massive explosion. Hmm. Maybe more than 'somewhat' subverted.
The Rurouni Kenshin anime has not one, but two onsen episodes. Both involving characters accidentally seeing each other naked.
Saber Marionette J had a Hot Springs episode, giving a chance for gay Hanagata to rub up on Otaru wearing nothing but a very small towel.
In Sailor Moon, Usagi and her family go to an Onsen. Then a monster shows up . . . then possessed Prince Endymion . . . and finally the other senshi. It ends with all the girls in the Onsen.
Saiyuki: Reload Gunlock has an onsen scene: Sha Gojyo is draped with a towel; Cho Hakkai is just an outline in the water (but what an outline!!)
Episode 5 of Saki has the characters going to one for their Mahjong training camp.
As well as the ending episodes of season 1, which invited every member of the mahjong team to join them before the next round started up, notably leaving behind all of the male characters.
Samurai Champloo has a few instances where the characters visit hot springs, including one where Jin finds himself in one along with an Ambiguously Gay samurai who intends to kill him.
Sayonara, Zetsubou-Sensei has an episode in which the gang decides to detox at a hot springs facility. Fanservice is mocked, the class of highly problematic students end up losing their quirks and becoming normal... leading Itoshiki to re-tox them with highly caloric food. He then gets pushed onto the de-toxing spring himself, then dramatically disappears. He was all toxins.
For bonus points, count how many times Kagero Usui can be seen on the girls side, mostly unnoticed by even the viewers. At one point, there are several Kagero Usuis all over the fence.
Played with in Sgt. Frog, where one of Momoka's poorly thought-out plans to get closer with Fuyuki involves letting him win a set of four tickets to an onsen, and hoping he takes her as a guest. Most of the action involves Momoka fighting with Natsumi's friend/admirer Koyuki over the extra ticket, to the point where they don't actually get to the hot springs until the very end of the episode.
Subverted in Sound of the Sky, since the expected (from the preview) hot springs episode ends having less than one minute of actual hot springs on-screen.
Sukisho: The last episode. Taking into consideration that it is more or less filler it is not surprising that fanservice should abound.
Just about every episode in all the Tenchi Muyo! series have at least one scene in an onsen. The first TV series has Washu build one into a Pocket Dimension attached to the bathroom - a woman opens the door, and enters an opulent onsen with steaming water fountains. A man opening the door just enters the normal bathroom. Ryoko builds the onsen in Tenchi Muyo! Ryo-Ohki. Lacking Washu's ubertech (or at least, lacking Washu's ability to invent new ubertech on a moment's notice), she just built a Floating Island behind the house and put the onsen there.
The OVA had two episodes actually dedicated to the place: Episode 4, which served to introduce Mihoshi to the cast, and Episode 9, which served as not only a sequel to episode 4, but also dedicated itself to Sasami's backstory.
The sixth episode of Tengen Toppa Gurren Lagann had the main characters directed to a hot spring by an old man. Kamina and Simon needed the concept explained to them, as both had grown up underground. The spring turned out to be not only a trap set by the Beastmen, but a Ganmen in disguise. Also, there was a wardrobe malfunction. Followed by an explosion.
This Ugly Yet Beautiful World had a hot springs scene where ALL of the series' female characters participated. Granted, nudity in This Ugly Yet Beautiful World is more plentiful than Ho Yay in Revolutionary Girl Utena, but the hot springs scene shows almost full nudity for all the characters, including the one nude shot of the female preteen character. Then the very next episode was a Beach Episode...
Tokyo Mew Mew has an episode where the girls take a trip to a hot spring. However, this one is fairly devoid of fanservice.
Transformers Cybertron manages to make use of this trope in one episode. Notable for it taking place in Alaska.
The Wallflower had an episode set in a hot spring, which featured a murder mystery, Sunako generally creeping out everyone staying at their hotel, and a truly awe-inspiring game of ping pong.
The Weatherman Is My Lover features an extra where the cast and crew of Chou Hayaoki TV have what amounts to basically a school field trip to an onsen.
WORKING!!: Subverted (or at least weirdly played). The hot spring episode does not feature any hot spring scene at all (or 1 if you count the foot spring).