A 20 gallon is not very large as you know so don't have too many high expectations.
Having said that, some of the simplest fish to keep are also the most active and colorful... I'd suggest a trio of platies, a trio of cherry barbs, 4 dwarf chained loaches, 1 male banded gourami (colisa fasciata) and 4 male endler's. That would deffinately be active and very colorful and would render your tank fully stocked. Planting the tank quite heavily would be agood diea as well as you'd have quite a few fish and they'll want to have hiding places/territories etc.
If you want more boisterous slightly larger, fish, perhaps a pair of bolivian rams, 5 praecox rainbowfish and a couple of female mollies would be better. (You have no idea how difficult it is for me to suggest a community tank without gouramies

)
Or you could try something with lots of tiny fish. They'd be small but their number would mean there's always activity: 5 pygmy cories, a trio of honey gouramies, 7 spotted rasboras and 4 male endler's are what I would suggest.
If you'd like to try something
completely different, try some neolamprologus/lamprologus multifasciatus shell-dwellers. they're tiny but worth it for their remarkable behaviours. You'll need to research them carefuly if you want them to do well as they have a few specific requirements.
There are plenty of other options. I'd suggest going to your LFS, taking note of any particularly active or interesting, but small, fish and coming back to research them and see if you could keep them.
edit: Don't attempt angels in a 20 long.