There are a lot of classic, inexpensive reef fish to choose from, ideally you want some that will inhabit all levels of your tank from the substrate to the open water at the top. It would help quite a bit knowing if you plan on getting a lid or not.
I'd look into a goby and pistol shrimp pair, I love my yellow watchmen and tiger pistols. But there are several species of shrimp-pairing gobies you could go with. If you want a more utilitarian option and don't want a pistol shrimp, then a diamond watchman goby would help keep that sand bed clean and stirred. Definitely need a top for a dwg though.
I'll always recommend clown fish based on their personality, ease of care, and now sheer variety of patterns.
Banghaii cardinal or pajama cardinals are peaceful fish that will hang out in open water.
A firefish is a fun addition that really stands out with it's unique swimming motion and beautiful coloration.