According to Hot Deals Club:
Staples.com - HOT! - $10 off $20 coupon online & in-store
Click the above link, and click to play the Desk Apprentice Game. Play the game for a few minutes, and afterward they will reward you with a $10 off $20 Staples one-time-use coupon valid online and in-store. The personalized coupon they send you will expire May 31.
Update (11am EDT): Their site is running very slow, but it still works. Just play the game for a few minutes and you will get the coupon. Just as the Staples trademark says, "that was easy."
According to Reseller Ratings deals:
Staples has a promotional online game with the prize being printable coupons for discounts on retail and online store purchases. Most people seem to be receiving a $10 off $20 coupon, though not all coupons may be the same.
I've gotten my score up to the top level, "Trump," but there's nothing about a coupon.