There are a few games you can play with a whiteboard:
-One that always goes down well is Pictionary. Two kids are given a board and try to get their teams to guess the song they are drawing. - Guess that word: You send one of the children outside, write a word on the whiteboard that the kids can't sing and the child then comes in and has to try to guess the word they're not singing.
Well actually that's all I can think of for now. I'll add to it later if I think of more :)