dwarf puffers are notorious fin nippers and do best in a species tank you could have 3 dwarf puffers in there. cory cats are cool and easy to take care of you can mix different ones and they will still shoal together. my ten gallon has a betta and two julii cory cats and one albino. you could do a few cory cats and a centerpiece like a betta or dwarf gourami
Google nano fish as they all work great in a 10g. Something like Badis Badis and Red Cherry Shrimp makes for a nice looking tank. Endlers are one of my favorite nano fish. We stock all males in our Fluval Edge 12g. They are very colorful little fish!
Endler's are colorful if you get all males. All male guppies would work also and come in a lot of colors. I'm not fond of platies in a 10g only because they have a higher bio load and you can only but a couple in.