I just started a variation of the 30 day squat challenge! Yesterday (Wednesday), I did 50 squats, 20 situps, and 20 pushups. Today, I'll do 55 squats, 25 situps, 25 pushups; tomorrow I'll do 60 squats, 30 situps, 30 pushups. The day after that, I'll rest. Sunday, I'll resume with 70 squats, 40 situps and 40 pushups.
So the pattern is increase the reps by 5 each day for 3 days in a row, take a rest day, then increase the reps by 10 after a rest day. You finish the challenge on Day 30 with 275 squats, 250 situps and 250 pushups.