Those are definitely some great suggestions above. IME astreas do need to be uprighted if fallen over, but that generally doesn't happen that often. I've had about 8 for two months and in that time, only one has fallen over. I uprighted him and he was back on the glass eating algae within minutes, so don't be discouraged by that fact. I think they are great snails and make a great CUC. Trochus snails are also great snails.. but are harder to find than the more common snails, such as the ones listed above.
I will however warn you of nassa snails. They are big monsters in little packaging. I had a few of them and they destroyed, tore apart, and ate alive, my perfectly healthy feather dusters. All three of them. They also climb on rocks and nocked over coral in my tank.. so i can honestly say that i will never ever buy them again. I keep my sandbeds under 4" from now on because of them haha. Still, i've heard other people say grea tthings about them so it ma have just been the ones that i get. If you do plan on getting them, i would suggest buying the smaller nassas, the ones that only get about half an inch. The bigger ones which i purchased may only be the ravenous monsters, and not the smaller ones.
Cleaner shrimp are by far the coolest and most active shrimp ever. If you stick your hand in the tank to place coral, or move around rocks, etc.. chances are that shrimp will be climbing all over your arm and hands. The one i had used to pick at the cuticles on my nails when i was placing coral. Even if i moved from one side to the other it just sat on my hand doing it's thing. So they're definitely a plus in any aquarium.
Also, some awesome crabs are the emerald mithrax crabs. They are just so much fun to watch, like any other crab im guessing, they just pick food up and shovel it into their mouths and then spit out scraps they don't want. It's hilarious. Plus, if you get close to the tank they wave their claws at you and if you don't move back they retreat in to rock crevaces. So funny.