I have my participants pair up and play rock, paper, scissors, and then have the winners do one thing and the losers do another.
Usually it looks like:
Very easy to make this easier or harder by altering rest times in beween, altering how long the work period is, or by challenging higher gears/speeds for different sections.
This is a great way for people to get a laugh, get to know eachother, and kinda forget for minute that they are working out!