As mentioned a fire belly toad would be a good candidate.

You would also be able to successfully keep 2-3 spring peepers in there, or 2-3 chorus frogs (depending on the size of the subspecies you choose). They are fun frogs to keep and would do well in a 50/50 setup. Cricket frogs are another option. You could keep 3-4 cricket frogs in a 10 gallon just fine. All of the species I mentioned (aside from the firebelly toad) stay quite small.

Don't mix species, either. That is never a good idea with amphibians.