Beautiful background art was practically mandatory in cartoons from The Golden Age of Animation, so there are literally thousands of examples to choose from. To help give an idea, there's an entire blog dedicated to recreating backgrounds from these old films.
Code Lyoko should be the posterchild for this trope. The lead background painter by the name of Frédéric Perrin created meticulous backgrounds which were utilised in almost every scene in the non-3D sequences of the show; Indoor, Outdoor, Industrial, Urban, Nature, you name it. Check out his work for yourself.
The animated series Dungeons & Dragons features quite a bit of Scenery Porn, especially when showing what formidable landscape the teens had wandered into that week.
Hanna-Barbera might've had low budgets for their cartoons but many of their late fifties early sixties cartoons have beautifully painted backgrounds. John Kricfalusi talks about the paintings here.
The cartoon To Spring, which also has lots of Technicolor Porn with all the garish paints.
The Simpsons: "The City of New York vs. Homer Simpson" has a few examples of this, including the obligatory leaving-Manhattan-via-a-bridge ending (and "camera" zoom out).
The Teenage Mutant Ninja Turtles (2003) episode "The Ancient One" is full of these, once Leo arrives at the hidden land, where the backgrounds, instead of being in the series' usual style, are painted in a manner reminiscent of Avatar: The Last Airbender.
While not exactly unimaginably detailed, the backgrounds from the Looney Tunes Wile E. Coyote and Roadrunner shorts were◊ very◊, very◊ pretty◊ and had gorgeous colors.
The House of Mouse Prop Room (an extremely large basement that's supposed to house different props and backdrops for (almost) every animated Disney movie ever made), which for some reason, looks like something drawn by Mike Mignola of all people! Guess which Disney movie Mignola was involved in!
The French animated series Wakfu takes a few cues from anime... including gorgeous artwork. While this devArt page offers more characters than background, those backgrounds you do see with the characters are the barest sample.
Of all shows, Family Guy can do this when they want to. It's usually when they're trying to make the setting romantic or just pretty, for whatever reason. A more specific example would be the walk on the beach Adam West had with Lois' sister.
Thundercats 2011, is flush with many exquisitely rendered, unique environments jam-packed into each episode, detailed (and spoiled) here.
My Little Pony: Friendship Is Magic manages to pull this off in its Sugar Bowl of a Flash cartoon (take these two◊ shots◊ for example). The color, fluidity and amount of tiny details crammed into general shots are impressive. Some episodes use backgrounds that have never been seen before and are unlikely ever to be seen again, like the Training Montage in "Call of the Cutie", the fall foliage in "Fall Weather Friends", and the Wild West scenery throughout "Over a Barrel". There's also the pegasus city of Cloudsdale, an entirecity made of clouds and rainbows. Its a pretty Sugar Bowl! In fact, Cloudsdale◊ looks very similar to Mount Olympus◊ from Disney's Hercules (which also falls under this trope).
Motorcity manages to make a desolate, run-down, subterranean Detroit look absolutely gorgeous.
Coupled with a kickass Instrumental Theme Tune, Gravity Falls has one of the most beautiful animated intro themes for any cartoon show in recent memory, especially considering that it's from the Disney Channel. Even the animation within each episode conveys the show's setting in the Oregon countryside to a tee.