I would recommend doing a timed circuit. Spend 30-45 seconds on each station then take 15-30 seconds rest in between. Mix some body weight exercises with some running, battling ropes and kettlebells, sandbags, TRX or punching moves. Also, if you have any stationary or spinning bikes available add them to the mix. This will get them going and make it more fun as well. You can only do so many squats, pushups, burpees, jumping jacks, etc. I would do 8-12 stations based on the number of participants and equipment you have at that time.
Always add something completely different in the mix and just watch good things happening. Each participant works at her or his pace and doesn’t get intimidated by others who can perform a number of repetitions faster than them.