Definetly look at Trochus, these guys are about the size of a margarita/astrea, they can handle our tank temperatures, they reproduce in a good tank environment (I have 2 babies in my tank with 8 adults) and they can flip themselves over when they fall on thier backs (unlike margaritas/astreas).
Cerith are good for stirring the sand as thats where they retire when they arent grazing.