Clowns are super cute, but you will probably get more info on them if you post a question about what clown work in a 10 gallon in the saltwater section. Personally, I only keep fw
tanks, so I can't be of much help there.
The crabs that I know of need brackish water and access to land/air to feed. You can certainly build an awesome little crab set-up though.
Another option would be African dwarf frogs.
If you want fish, all the fish AquaKai suggested work. I have a 10 gallon with a betta and ember tetras. Bettas are actually fairly active once you get them out of that silly cup that most places sell them in. And, they are very colorful. Guppies and Endlers Livebearers are another option.