Is there anything you would recommend stocking a 2.5g tank with besides a betta? Maybe guppies? But how many?
Oops! I meant to put this in Gen. Freshwater...hehe
Is this going to be a planted tank? IMO, small tanks can look stunning with live plants and I think plants help improve the environment in small containers, keeping the fish healthier.
I agree with Tanker; you could try a couple small shrimp or an African dwarf frog as well as considering fish. A female betta with a couple Cherry shrimp would make a fun tank