We're going with a company that offers these rewards but you could do it on your own:

each kid that sells five items gets a lunch bag with the school name on it

each kid that turns in their order form, turns money in early, sells more than ten items...gets put into a drawing where they win an ipod.

I like this because even the kid that sells one item as a chance at winning a nice prize. Top student in each class/grade gets something as well as top student overall(this child doesn't win twice). Hopefully our kids will like this and do well. I like them over past companies because in addition to all the seasonal stuff they also sell sports items which will be a hit here.