Here's a couple thoughts that I had for my 10:
Crabs- This would have to be a species tank and rather brackish. It would also have to be only about half filled since they need to get out of the water and are escape artists.
Crayfish- also escape artists and generally don't play well with others so another species tank.
African Dwarf Frogs or Newts- could be fun. Newts need to get out of the water though so its a bit more of a terrarium thing.
Betta with some cories- I have this in my 8g hex right now and love it. Its a great tank since the bettas have such a personality. The cories are fun too.
Dwarf Puffers- this is what I'm going with. They're cute, small and should give me a place to dispose of the pond snails from my other tanks. Again, a species tank though. Qualified tank mates for the these guys are limited from what I hear.
Tom