I've had all the normal stuff: britsle worms(one that was almost over 7in long), spaghetti worms, pods, bristle stars, the crazy multi-legged little blue stars, a rock crab or two, some really cool snails, etc.
I did have some hitchhikers on a zoo rock I bought. They were little slug/nudibranch looking things that were brown with little soft spikes coming up off their backs. They spent their days wandering around the zoo rock that they blended in with.