A variation to test your natural sense of rhythm:

  1. Start the stopwatch.
  2. Watch the counter for the first 10 seconds.
  3. On 10, close your eyes and try to keep the rhythm going in your head.
  4. Attempt to stop the watch exactly on 20 seconds.
  5. Give yourself a reward or forfeit according to how close you are.
  6. Try your friends. (Although if this is your idea of entertainment, it's unlikely you have any)

I am reliably informed (thanks, shimmer) that another version of this is to stand in a group facing a clock. On a signal, turn around so none of you can see the face, and you each have to attempt to sit down after exactly one minute. The closest to one minute is the winner.