This is for the WHOLE CLASS! Just buy a regular old cookie sheet and attach magnet tape to the back, and then stick it to your white board. Anytime the class as a whole are doing a great job by listening, following directions, being responsible, finishing their work, working quietly, etc.... you can give them a "brownie". When they fill up the whole tray they can pick from the mystery envelope.
Restore dry erase boards that are hard to erase: spray a clean board with wd40, wipe dry with paper towels. the wd40 fills in the dried pores of the board that hold in marker ink, making it easier to erase.