Cherry barbs... they are nice and active like the danios, but they will add some color. Platies or mollies or (if you have a very good cover) swordtails will all get along fine with danios. Pretty much anything you see listed as "community" should get along with danios, especially since you've got enough of them so they shouldn't pester other fish. Check out your local stores and see what's available that you like, then do some research to make sure they will fit in a 20g and they will work with danios.
A male betta might be okay, since you've got enough danios that they should just bother each other, but then again they might decide to go after his tail. A female betta would definitely be fine in there (without a male). They are just as colorful, but they don't have the long tail to tempt other fish.