I agree with h20 plant about the dwarf puffer -- mine is alone because he ate baby shrimp, harrassed snails (and ate small ones), and harrassed the occasional fish fry that I put in there to clear out planaria and such. Not a good community tank member at all! But he is interesting to watch if you don't mind just one fish in the tank. And, of course, with 2.5 gallons, you're not going to have many fish, regardless of what species you choose.
Other options might include cherry shrimp and maybe a single pretty mystery snail (pomacea bridgesii) or a few small fish like Endlers. The 2.5 gallon is a lot of fun, but it can be a challenge to keep a tank this small balanced and to work with the limitations of just 2.5 gallons (having to choose few, small fish, etc.) But it's an enjoyable challenge!