Each person brings two exercises written on individual scraps of paper. Put the scraps in a bucket. One exercise is then drawn from a bucket at random.
That exercise is then subject to a tabata set -- 8 rounds of :20 on, :10 off.
Draw another scrap of paper and do it again, for as long as the group can stand it.
UPDATE: We did 5 tabata sets, with the following randomly drawn exercises:
• Squats w/ barbell
• Hand-release pushups
• Russian twists [ demonstrated here ]
• Kettlebell swings