If you had a sandbed deep enough, my first thought would be to do a tank for yellow head gobies, you know, the ones that burrow in the sand. Those fish are so comical, especially if you have 2 in there as they will steal shells and small bits of rocks from each other.
Another choice would be a damsel tank. I realise that most people consider them 'disposable' or 'unworthy' of a salt tank, but they are very interesting fish that if you were careful in your selection, would provide tons of color and lots of movement to your tank. (Kinda the salt equivalent of an african cichlid tank).
Some people find mantis shrimp interesting, and there certainly are some very colorful kinds available.
An octopus tank, although you would probably want to reduce the amount of LR
if you did this or you would never see the animal.
Seahorse tank, although again, you would probably want to reduce the amount of LR
, and remove the chili coral too (I believe that it will sting them when they try to hitch to it).