There are tons of options at that size! Keep in mind tangs are out of the question, don’t even think about it, but beyond that, here are some of my favs:
clownfish pair ( can’t beat a classic! )
Six line wrasse ( great color, size, and a little utilitarian )
Fire fish ( some great color and an interesting shape )
Skunk cleaner shrimp ( speaks for itself, utilitarian, and beautiful )
Pincushion Urchin ( fun and unique invert that can even help you detect if something is wrong with the water! )
once again, tons that you could stock a tank of that size with, just some things to look into!