Here's a really great game with tons of math going on behind it. Poison has a simple basis of removing counters until the last person has to take the Poison. Its something your students can play if they finish early, or are waiting for you to set up for small groups.
Let them play a lot before you hit them with the challenging part. Ask them to try and explain a strategy they have for winning the game. Can they explain it mathematically? Their answers should get very interesting!
The kit includes a game board, a function table, and a strategies sheet to help your students explain their thinking.