A blenny is a good choice. The tail spot, midas, starry, or lawnmower blennies would all fit in this tank. They're good for controlling algae and have wonderful personalities. Very active.
A watchman goby/pistol shrimp is a very popular duo but always a good choice. I recommend that candy cane pistol (Alpheus randalli) as they stay smaller and look prettier than most of the other options. In terms of goby, the Randall's prawn goby is the shrimp's natural pair, but other shrimp gobies, such as the Yasha or Yellow Watchman Goby, also work.
Finally, there a ton of cool wrasses that would work well. Melanurus wrasses are very pretty, and you have a nice sand bed, which they sleep in at night. There's also the flasher wrasses like the McCosker's Flasher and others that would be beautiful. Wrasses are beautiful and active fish and you should definitely consider one.
You can also look into anthias, chromis, and hawkfish. All are very interesting species.
