The most common are squat challenges, plank or ab challenges.
I’d keep it super simple to help adherence.
One thing we’ve done has been one rep per day of the month: like with squats, 1st day = 1 Squat, up to day 21 =21 squats.
I would say it depends on what equipment is available, who you are trying to market and ultimate goal.
Get creative and good luck.
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