Here are a few ideas that I have used in the past...
1. Bring a snack on Friday
2. Play smartboard games (subject related)I may have it planned but they think it a reward
3. I put a big envelope up with a question mark on it and they will each get one of what is in the envelope...(candy, or some sort of pass soda homework sit by friend)getting to the suprise is half the fun
4. Computer Lab time...I'll plan a webquest and tell them their reward will be the last 10 mins is free time
5. Outdoor classroom...We have an area where I teach from, but they love to be outdoors!
6. bragging rights...I'll go over the announcements in the AM and let announce that they beat me!
7. Bonus points
8. work quarterly toward getting a big day...if they get more

than

in a quarter than they might get a recess, computer lab hour, munch and read, etc.