Here's what I've tried in my tank and have liked alot:
Hygrophyla: This one grows like crazy, but has been super pretty. I've been calling it the parsley plant because it resembles the herb.
Ludwigia: This one is so pretty, and does extremely well in the background with plenty of light. Mine has climbed out of the tank though, so that may not be something you like.
Aurea: This one is pretty tolerant, but looks great bunched up. Again without a enough lighting it will climb out of your tank, but it still looks nice, you just need to trim it. In my tank it's kinda gotten smothered by the parsley plant and ludwigia, I will try to save it before it dies.
Dwarf Sagitterius: Awesome foreground plant, that's been doing wonderful in my tiny tank. Grown a new head since I got it.
Others: Anubias, Moss Balls and Java Moss (though I'd suggest a finer moss for your wall (maybe christmas moss or willow moss) I've had these since the beginning and all are doing fine
. Take a look at my thread for my pico if you want to see how they look. It will of course be different in a 20g. Oh and I'd highly advise against any cabomba species.