The Stars are there as a beginning way for kids to chart their progress. Some ages may be excited about coloring them in each time they break their record. Have them write what level they are in and then give it a fancy color.
If you are at an upper grade you may want to do it differently, for instance just use it as a chart to show where they left off. Your reward in the beginning is just the satisfaction of breaking their record. As you progress, you may want to read the ebook on the Power Student Olympics. In that we talk about other types of rewards that you can use. Be creative...but not too spendy!