For my Early Elementary age students I have implemented a Reward System called "Sticky Treasures". On my wall I have a a chart filled with vinyl stickers (that students absolutely love). I tell my students that at the beginning of each class I am on the lookout for three students who really impress me – they are on task, kind, listening, following instructions, going above and beyond, etc. These three students will be rewarded at the end of class with the opportunity to choose one sticker from my "Stick Treasure" chart.
I try to rotate the students I choose as best as I can, really paying attending to how students are behaving throughout class time.