This is another, actually relatively common, dungeon type that still makes for a great change of pace: A treetop village. Probably the most famous example would be the Ewok village from Return of the Jedi, but they’re really all over the place, especially since many authors like to use them for Elves. The Channelwood Age from Myst is also an example of this.
But Nils, you might say, a village isn’t a dungeon! And you’d be right, for any normal village. A treetop village features the same limited movement than an underground dungeon does (provided your party can’t fly). Sure, characters could try to jump across chasms or improvise rope bridges, but that’s the sort of drama and problem solving that makes an adventure fun.
Why go there?
I think an “intact”, that is inhabited, treetop village doesn’t work well – the inhabitants will be able to communicate easily and mount an effective defense; our intrepid adventurers would fight wave after wave of defenders. Fun, but not your usual dungeon crawl. It also means any sort of hostage rescue is out of the question – the occupants could just kill the captives at the first sign of trouble.
A better approach is probably to use an abandoned treetop village. Maybe oversized spiders or some other wildlife inhabits it now. Maybe a villain on the run is hiding in it. And maybe the locals simply left something of value behind. Druidic artifacts or some other form of nature magic probably works best, but just because these guys lived in trees does not mean they did not like gold.
Who’re we fighting?
Any sort of animal or monster capable of flying or climbing. Giant spiders, semi-sentient vampire bats, twig monsters. If it has a place in a forest and can make it up, it can be a possible monster in a treetop village dungeon.