20 gallon suggestion:
1 dwarf male gourami (any variety)
3 corys (any variety 2.5" adult size and under)
6 zebra danio
3-4 MALE guppies
oto's if you can get 'em.
a handful of ghost shrimp.
The danio's will provide tons of movement. They're hyperactive little snots, with really pretty silver and blue stripes.
The corys are bottom feeders, though they'll often zip up to the top of the
tank or wander up and down the glass. I'm partial to cory. sterbaii for their
coloration, size and their more active daytime habits. But they aren't always readily available and usually run $8-$12 each.
Oto's are cute, get along with everything, and are good algae eaters.
The dwarf gourami will be your centerpiece fish, being the largest and most colorful fish in the tank. If you can find a matching female, consider getting a pair. Two males in that small a tank would be a problem.
The shrimp are dirt cheap, fun to watch and help keep the tank clean. Don't be suprised if 3 out of 4 die--they're feeder critters and have a high dieoff ratio, but the ones that survive the initial week should do great from then on.
Guppies? Not my favorite, but they can add alot of color, are inexpensive and reasonably hardy. Only buy males to avoid creating a guppy farm.
Platy's would probably also be a good choice.