Something small and peaceful. I would suggest reading info on "natural aquariums". Plants complete the ecosystem in my opinion. Some people do not want plants because of all of the work involved. It's kinda like taking care of a lawn. Lol. Nonetheless, I believe the fish live longer, healthier lives with plants.
Suggested fish: neons, rasboras, guppies, 2 Cory's. Do not overstock. Less is more, but purchase at least 6-8 neons (schooling fish).