This may be a silly suggestion (won't work if you plan to stock any legit big old predators), but have you considered a yellow stripe clingfish/goby? I have absolutely fallen in love with mine because even though he's dinky he's ALWAYS out and about visiting with me. You could also potentially consider a 'reef safe' dwarf angel, just be careful about it! Also, nthing the blenny/goby sand sifter idea, those little dudes can really be full of gumption. Even though they might look plain-ish, the yellow watchmen do really pop against sand. In addition, bar gobies in my experience really tend to be 'out there' in the tank if there's nothing aggressive, and they are quite interesting to look at. I'd also suggest anthias if you don't like the suggestions thus far...if you're careful in choosing, you can avoid the issues associated with other groups of fish and still get a very colorful little tank guy!