Sexy shrimps are awesome in that size tank (my boyfriend has one in his 40B and loves it) but beware wrasses can pick them off and eat them. They really like to host anemones if you have any which is cool to watch (ours hosted a bubble tip until the clowns found it then he founded the rock flower nems). I'm pretty sure they'll be fine without though. They're the only shrimp my boyfriend likes because if you feed them they don't go out of their way to stead food from coral.
Pom pom crabs are one of my favorites. They're really cool looking and fun to watch. However, the vast majority tend to be nocturnal and only come out at night (mine came out all the time so this not always true). They're pretty small so may also need to watch with wrasses with them.
I love my new blue tuxedo urchin, he picks up everything (sand, shells (sometimes with snails still in it), small rocks, thermometer,ect). It's so much fun to watch. I actually go out of my way to make him frags of zoas for him to wear so that he's always colorfully decorated. They supposedly eat a wide variety of algaes (including many nussiance algaes) and even corralline algae.
Every now and then I even buy one... like the crab.
