Great visual whole-class incentive...you can adjust the size of the brownies to make incentives easier or more challenging to earn. You don't have to reward them with actual brownies--let the class suggest incentives they're motivated to earn!
Class Incentive Project with Potato Head from AlstonDesignStudio on TeachersNotebook.com - (6 pages) - Class Incentives project. Use as a way to measure / document class goals to earn rewards. When the class accomplishes goals or behaves well, the teacher adds a part to the potato head.