If in general, you want fish and just don't want to use a large tank, I think a trio of neons could live in a 5 gallon pretty well. I've got 4 in a 10 gallon, and they only use the bottom inch and a half. ^_^

Probebly NOT what you are looking for, but until they get big (and most die before they do) you could probebly keep a ghost shrimp or 2 in a 1-gallon bowl, correct me if I'm wrong. You'd just have to make sure there is a lot of gravel, so the floor space is near the widest diameter. Personally, I love watching Ghost shrimp. There is always the chance that they'll grow, but i've never had one grow even to an inch in length.