Go for the shrimp. I have 3 pea puffers in my 8 gallon & can't imagine how well it would work in a 2.5 gallon. Keep in mind I have random plants scattered & floating *not scaped really* so they can explore & also to get rid of there aggression
Another vote for shrimp and snails. My original plan for my 3 gallon was shrimp & small fish, but I added the shrimp first to let them grow a bit. Now that I've had them for a while, I've decided to keep it just shrimp and snails. The shrimp are so active when kept alone that you don't 'need' the fish, and their load is so small that a bunch can live and breed in there just fine with less maintenance than overstocking it with fish.