Sometimes I think people go overboard with their CUC. +1 for nassarius snails. They're detrivores and will clean the substrate of waste and excess food. Plus they're fun to watch when you feed the tank.
I'm not a fan of hermit crabs, though. They irritate my coral, but they are a nice addition to the tank and CUC, nonetheless.
Emerald crabs seem to be a staple in most CUC. Voracious algae eaters, even hair algae in my experience.
Turbo snails get huge. They act like bulldozers. I try to stay away from them myself.
If your tank is mature enough, an urchin is a neat addition. As slitherbomb stated, it will eat coraline and other algae. You can supplement with nori wrapper attached to a rock or something.
Sometimes, less is more. Experiment and see what floats your boat-that's the fun part anyway.