A pair of fish from the apistogramma genus: Care must be taken to provide plenty of hidy holes as males are aggressive toward females for the first few days. You also have to be prepared to pull out the male if they spawn as the female will often kill the male in a small tank in an attempt to protect the fry (this happens very quickly and the male should be removed BEFORE the fry go free swiming). He could be temporarily housed in your 20g. A few pencil fish and otos can be added for algea control and as "dither/target" fish.
A pair of small Juliochromis (J. ornatus or J. transcriptus): Tank should have abundant limestone based rockwork, and care should be taken to maintain a high ph (accomplished be using aragonite/crushed coral as a substrate or including some of this material in the filter). Additional addatives to the replacement water can be used, but I find that smaller (20%) more regular water changes eliminate the need for these depending on chem of your tap water. A rubber pleco can be added for algea control and should be able to adapt to the high ph and hardness.
A pair/colony of Tanganyikan shell dwellers: Same water chem concerns as above, but more open space and lots of empty apple snail shells.
The only one of these setups I have not personally tried is the julies, but based on my reading they should work well.
Other possibilities include Laetacara cuviceps, Dicrossus filimentosus, and Keyhole cichlids. Keyholes get bigger (4in), but seem to be pretty sedate swimmers (more like apistos than the more hyper rams) and a single individual should make a good centerpiece fish in a 10g. A single ram could also be kept as a centerpiece. Lots of stucture on the borrom should make up for the fact that a 10g is a little on the small side for these active swimmers.
Good Luck