These are some of the things I do to add some zip to doing sprints.
Rock Paper Scissors - Divide the class into 2 teams, and have one person from each team go head to head in Rock Paper Scissors. The losing side sprints for 20 secs. The law of averages usually makes things work out, but if not, I add "sympathy sprints" for the winning side, etc.
Partner sprints - another instructor showed me this one - pair the class up and have person A sprint until they can't go any more (pick a tension such that 25 sec is about the max) and then partner B takes over - going back and forth for one song length. Very intense!
Cheers,
Mike