I would do a small school of blue/green chromis, awesome fish to watch school together through the tank, completely reef safe later on, and they're hardy little guys form everything I hear. My kid loves watching them swim back and forth at the LFS
. Possibly add in a royal gramma basslet and a pistol shrimp/watchman goby pair, awesome symbiotic relationship for your tank! You'll still be able to support a few more fish in a 60 gallon system but add slow so your bacteria can keep up with the bioload.