Some sort of South American cichlid would work well in a 20g. For example, a pair of blue rams or a pair of bolivian rams or a trio of Apistogramma trifasciata or a trio of Apistogramma cacatuoides...the list goes on, there are many apisto species that would work. Plus, these would go very nicely with cardinal tetras.
I recommend this as a nice South American themed tank quite often for 20g's:
2 Bolivian rams (or the other options I listed above, or other apistos you like. You can do some research into dwarf cichlids and see what catches your eye)
6-8 tetras of some sort
3-4 otocinclus (small suckermouth catfish)
1 apple snail
Plant it with amazon swords and other plants of your liking. Have some sort of floating plant. Add bogwood and a few river rocks to create hiding places and there you go. Hope that helps.
-Larissa