I would map out exactly what you want before adding too much more, especially in regards to fish. Any clown would do great in a 20, even a pair would be fine. Not sure if you're running a sump, but you'll want to make sure you don't add too many fish, your tank will not withstand the bio load. As far as corals... Zoanthids are a great beginner coral, you can get many for cheap, they're very easy to care for, and add a lot of color. You could also consider most soft corals: fingers, mushrooms and some LPS such as frogspawn, Candy Cane, maybe a torch. I would avoid SPS until you successfully house softies and LPS.