Surprising in a world of Jerkass Gods and excessive heroes, but one actually exists in Classical Myth with the story of Baucis and Philemon: after being the only people in their village to grant a disguised Zeus and Hermes the Sacred Hospitality despite their poverty, they are granted a wish for doing so. They wish to die at the same moment so that they would never be widowed. As if that wasn't enough, Zeus throws them something extra: when they do die, they are turned into trees with branches entwined forever to symbolize their love.
There's actually a fair few in between the usual bloodshed and weirdness. After all, at least some of those stories gotta have a Happy Ending, and the gods do Pet the Dog at times.
At least some versions say that the reason Zeus and Hermes were in that village was specifically because Zeus was pondering obliterating mankind and wanted to see firsthand if he should. After the first few houses rejected them, Zeus was somewhat understandably seething, but Hermes managed to convince him to give it another try in the hopes of finding a decent family. Repeat: Zeus, nigh-almighty king of the gods with the Hair-Trigger Temper and Bolt of Divine Retribution, listened to his son instead of getting even angrier.
Psyche, after enduring all manner of trials to gain back her husband, Eros, not only wins over Aphrodite's tasks, but is taken to Olympus to live with Eros forever, and their daughter is named Delight. Even more so, Zeus... y'know, frickin' Zeus, actually gets involved by approving of the immortality of Psyche and then personally telling Aphrodite to cut this whole crap at once on the request of Eros.
A rather odd one, thus fitting for Hermes: When Pan was born, his own mother ran away in fright due to Pan's lower half being that of a goat's. Hermes' reaction? He thought Pan looked awesome and took him to Olympus to show him off.
When Penelope and Odysseus embrace after ten years of his absence, it's described in terms of an exhausted swimmer who comes crawling back onto the warm shore, "in joy, in joy, leaving the abyss behind."
The fact that Odysseus loved Penelope (who is already getting old) so much that he chose her over immortality and a goddess for a wife.
Galinthias was the faithful friend of Alcmene, Heracles' mother. When Hera got Eilithyia (goddess of childbirth) to delay Heracles' birth, Galinthias tricked her into stopping before Alcmene was driven mad by the pain. Gods being gods, she was turned into a polecat, and then Hecate came along and adopted her as one of her animals. Sometimes even the Greek gods do random acts of kindness...
After Ariadne was killed by Perseus, Dionysus descended to the Underworld to bring her and his mother, Semele, back from the dead and made them both immortal.
Many versions of Hades and Persephone's story. In particular, there's the fact that Hades was probably the only Olympian who has never, ever cheated on his wife; the one time he came close to it, it was because the girl was trying to seduce him!And then Persephone came...
Most versions of the myth shows that Hades actually went to Zeus and asks him for permission to marry Persephone. So Hades actually went through genuine legal means to marry her, as oppose to the standard "I am a god so I can just rape you" that most Jerkass gods do.
In several versions of the myth, including a few hyms, Hades also apologizes for having kidnapped Persephone and hopes that she can forgive him for that. Unlike his little brother Zeus, Hades actually feels bad about his more questionable actions.
Several versions also point out that when Zeus sends Hermes to Hades with the message "Sorry but Demeter is making the Earth barren till she gets Persephone back so you have to return her", he is actually willing to do so (because Persephone insists as well), that is until they find out that Persephone cannot leave for good because she ate a bit of a pomegranate from the Underworld meaning she now belongs there as well, and is happy with the compromise of spending a part of the year with her and letting her spend the rest wherever she wants with her mother.
While far from perfect, Perseus was among the closest to a Nice Guy among the usually prideful heroes and semi-gods. He went "Medusa hunting" not for glory or wealth, but to save his beloved mother Danae from her Stalker with a Crush, who happened to be the king of the island they lived on. After Perseus punished said stalker via petrifying him as well as his court, instead of taking the throne for himself, he gave it to the fisherman who helped him and his mom — who happened to be the rightful king, dethroned by his brother aka the evil king.
Also, along the way home from Medusa-hunting, he saw Andromeda being bound to a rock to be killed by Poseidon's sea monster. He saves her not because he wants to bang that hottie, but because it's the right thing to do, even if he has to delay the Medusa head delivery. It's no wonder they become one of the rarest examples of Happily Married within the mythology.