Bring your own bag of lesson plans -- I've seen WAY too many teachers leave insufficient lesson plans. You can bring things that are multi-level, educational, and not necessarily what they do throughout the usual day. Like: several decks of cards (they can play addition games like 99, Go Fish, even concentration), a story book that can easily be turned into a drama (bring a few props) . . . If you will be there several days you can even bring them a little treat (oranges . . .) If you maintain authority but are still fun then everyone will like. Act like you know what you're doing and they'll never guess that you don't :-)