hermit crabs are excellent to keep they are actually very amusing little guys, i always get a laugh out of mine they make me think of martian marvin from looney tunes for some reason. go with one of them, they are very low maintenence too so they wouldn't be a lot of hassle like a frog. any aquatic frog in a 2.5g tank would need to be cleaned constantly. go with the crab or a scorpion, emeperor are the best as far as scorpions go in my opinion. if you like inverts you may want to get a fancy snail or make that 2.5g into a water tank for a dwarf albino clawed frog or blue crawdad, i have a 5g housing one of each of those and it has worked out great. good luck.
"Only through education do we teach the ignorant that which we love is not evil but wonderous"....