Wave is a hit at the school where I work. Mick did a twist on wave so the kids thought he was doing math. A kid would ask Mick a math question, and I'd signal him to wave to match the answer. Also- signals themselves are popular, too. The kids are amazed that your dog can do something without being "told", and, the kids can get the dog to do something (stand behind the kid and give the signal for whatever the kid is asking the dog to do).
A couple of others:
Depending on how much room you have, jumping through a hula hoop
weaving through your legs
marking and retrieving something