Here are my initial thoughts. How is your class engagement and overall behavior? If they're always on and doing well, the Scoreboard might just be for fun, and doesn't even have to have rewards. The reward could be beating you! Most of us are using the Scoreboard just to get basic control of the class. Since you're teaching something that is already fun and motivating, you may need to use it less intensely than some of us.
My first thought for an actual reward though would be that if they're already doing well, then most days you would be letting them win. Winning by any amount means we do business as usual. If they lose, then each point they lose by equals one less minute of game time. Maybe replace game time with listening to you read Shakespeare, as they clearly need a few more minutes of culturing that day?
WBT Intern 2011-2012