A boxfish needs at least a 50 gallon tank os that's out, but I think the clowns are a great idea! If you upgraded your lighting you could get an anemone and see if it would host them but they'll be fine without one. You could also add 3 or so shrimp, peppermint is a good choice. You can have less inches of fish per gallon in saltwater so the two clowns is all you'll really be able to get comfortably. Small saltwater tanks are hard but it looks like you're doing a good job!
Read her diary on the main marine chat forum, she is pretty much done with the aquarium, has clowns & anenome in it. She also has shrimps etc.. This topic is very old so I guess some people might think it's been made not long ago. Anyways have a look there, she lost the box fish and the mandarin due to ich