Computer Lab Behavior Rewards

I could really use some help coming up with free behavior rewards.

 I teach in a k-5 school. I have a behavior rubric and each class tries to earn the most points by having good behavior. A class can earn a total of 15 points each time they come to the lab. When the class reaches 75 points (about 6 weeks worth of classes) they earn a free choice day. This has worked well and most of the behavior problems have disappeared since classmates will monitor each others behavior.

The problem is I am losing too much instructional time when I give them a free choice day. We are on a 6 day rotation and when classes get out of sync it is hard to keep up with who has completed the projects. To add to that all special areas liked my idea and have turned the last Friday of each month into Free Day. Any student who has had good behavior in all special areas go to the meet in the gym and play games instead of going to their normal special area class. This takes another day out of my instruction.  Does anyone have any ideas of how to reward classes without losing a lot of instructional time or costing me an arm and a leg?

Thanks for your help.