Salt and pepper cories are my favorite. There are also dwarf cories and pygmy cories. Spotfin cories get a bit bigger but would still work in a 20. All of these little guys are rather sensitive. Wait a few months after the tank has been established to add them and make sure they are in shoals of 10 or more.
For a small tank a single honey gourami would be best. Dwarf gourami are prettier, but they also come with more issues. They can be more aggressive as well are prone to the dwarf gourami iridovirus, a wasting disease that is untreatable.
You will still need a filter in a planted tank. In non-planted tanks filters remove ammonia, particle waste, and provide water flow. In a planted tank most of the ammonia is taken care of by the plants, but you still need something to move water throughout the tank to prevent dead spots as well as remove large particle waste from the water column.
You might also find some larger tanks! Or neat bowls or things like that. Good luck! I hope you can find what you want!