12 gallon perfect fish

OK - did that subject line make you wonder?

I've now taken my goldfish to the local fish store guy who was happy to save
his condemned life from a lonely, 12-gallon tank.

I want to go out this weekend and buy some fish. More than one. Preferably
adding something interesting like, I don't know - a snail or frog of some
type?? I have a 3 yr old son who would love something like this. And I'd
like an algae eater - maybe an oto cat?

What would you all recommend? I know I can't have many & what I do have need
to be small...

Thanks - I should have asked this to begin with

Mary