I know everyone has their own reward system in their classroom, but something that works really well for me is music "star cards".
Whenever I catch someone doing really good (answering a tough question, behaving well, bringing in something musical to share with the class, etc.) I give them a "star card". It is just a piece of paper that they write their name on and put into their classroom bucket.
Then, if the entire class earns a green (or for those who do not work on a color system, do a really good job as a class) then we do a star card drawing. I draw out 1 -3 names (or more if they are my tough fourth graders and have the best day ever). The names I drew get to pick a prize out of my prize bucket. My bucket is usually filled with hershey kisses and starbursts.
The system works great for me, because it lets me rewards good individual behavior and it also keeps the entire class doing well.
All of my grades loooooooooooove to get star cards. They go crazy for them.