I'm almost embarrassed to call this technique a challenge because it's so darn easy. I'm afraid you'll think, "Hey, if I wanted some lame exercise advice, I'd read Men's Health," but bear with me for a minute.
The beauty of this "challenge" lies in its simplicity. Because it's so easy, it's infinitely doable and, given that you can do it week after week, infinitely sustainable. Despite its apparent ease, though, it works really, really well.
100 Reps Every Day
Just pick one bodyweight exercise – one that correlates to some body part or lift you want to improve – and do 100 reps of it every day.
For instance, if your bench press is a little shaky or has stalled out, do 100 perfect push-ups a day – in as many sets as necessary – for 7 days. You don't even need to do the sets in succession. You can do one or two sets before your morning ablutions, another later on in the day after you sweep up the kitty's hairballs, and a last one while Serbia and Iceland are battling for World Cup glory.
It's an absolute certainty that your bench will be a little sturdier or a little stronger after those 7 days.
36,000 "Bonus" Reps Per Year
Once that challenge is over, start another one. If your squat is lagging, do 100 walking or stationary lunges (50 per leg) every day for a week. Even 100 full-range bodyweight squats would work, but feel free to do them goblet style holding a small weight if the bodyweight thing is just too easy for you.
You might follow that up with a week of single-leg Romanian deadlifts to shore up your glutes and hamstrings, or 100 burpees a day to strengthen the ol' ticker. Keep doing the challenges every week. It doesn't matter if you do the 100 reps in 10 sets or 1 set.
As they say, you're only limited by your imagination, and maybe your slothfulness. After a year, you'll have added over 36,000 reps to your training, and you're loopy if you don't think that'll make a difference in how you look or perform, even if the reps are low resistance.
Try This One
One of my favorites is side step-ups, using a milk crate, step stool, or your little brother's back.
You could even get away with doing it on an ordinary stair, provided you make one modification: Instead of touching your foot to the floor, touch your heel lightly to the floor and start the next rep. This prevents you from pushing off with that leg and makes the move a helluva' lot harder.