Pajama day, backwards day (wear clothes backwards), hat day, tie day, crazy hair day, crazy sock day......
Depends how old they are. When DD was in lower elementary they earned "bucks" that they could spend on toys from the treasure box or save up for something bigger. They could even combine with other kids to buy something together.
There was a long list of things they could buy including lunch with the teacher (a reward at that age!), receive a letter fromt the teacher, movie night at school, sit at the teacher's desk, have a guest come in to read a story, play a game - I can't remember them all. It takes some imagination but the kids loved it.