id say a betta, an african dwarf frog (make sure its not an african clawed frog), or a few shrimp - which i might add, I think, are pretty fun little guys. Theyll swim around and their little feet are going about a mile a minute - pretty hilarious.
On the betta note, I'd agree that a min of 20gal is a little extreme. In bettas' native environment, they arent in much water at all, as they flop from "puddle" to "puddle" in the rice fields and such. So, in my opinion a 1gallon is OK, not great but OK, although, a 1.5-2.5 is definitely better. My betta is in a 1.5 and he is as happy as can be, making bubble nests and playing in his plants. So, a 1gal would be OK for a betta.