A fun way to ask a boy to a Sadie Hawkins dance is to give him flowers and go down on one knee when asking him if he would like to attend the dance. Another fun method is decorating his locker and asking him through a note in the locker.
Another option is to give him a balloon with a note inside and the words "Pop Me!" written on the balloon. A more sinister, but still fun method is to kidnap and ransom one of his favorite items and return it only if he agrees to go to the dance.