have you considered African Dwarf Frogs? There alot of fun to watch. I got 2 and I keep them with a variety of fish. Its usually 1 frog per gallon but you could maybe fit 3 in there with proper tank care.
Yeah, it really depends on the tank dimensions. I can think of a couple species that could live in 2.5 gal, but they are all a real pain to find, even online.