Here's a quick and dirty little project to spruce up a dungeon - an enchanted garden full of mysterious and arcane fauna that should give the players some interesting things with which to interact. Some may want to kill the characters, some to talk to them, and they may be perfectly harmless. Not knowing whether or not they are poisonous, beneficial, or even intelligent plants is all part of the charm.
Perhaps some of these plants have healing nutrients and can be eaten, but it's a much better bet that at least one of them thinks the same of the characters. Either way, even if they are all mundane, a room like this is an easy feature that should occupy players for a while - at least until the dungeon denizens catch up to them.