Look up some of those old time strong man training books. Crap, I used to have a link for them. They have some crazy bw exercises. Here, I'll give you a short list of ones that don't suck (ie. are hard):
Regular Pushups
I think you can figure these out
Diamond Pushups
aka "triceps pushups" with your hands close together. They touch your xyphoid process at the bottom of the movement
Alternating pushups
Assume a normal pushups position. Leaving your hands in place, move your body over your right arm. Go down and up. This puts more stress on your right side. Now move your body over your left arm. Go down and up. This puts more stress on your left side. Alternate like these each rep. When it starts getting hard, only do the eccentric portion (going down) while leaning over each hand, and when you're going up do it centered. This will let you get a few extra reps.
One handed pushups
The normal way with one arm under your chest and your feet spread. Non working hand goes on your thigh or behind your back if you're a stud
Sideways one handed pushups
These are harder to describe. You support yourself on one arm with your chest/stomach facing the wall and your back is facing the other wall (so you're rotated up 90 degrees from "regular" pushups position). Keep your body straight. Lower yourself by bending your elbow. Go as low as you can because you have insanely bad leverage on these and you probably won't get all the way down. I should just do an MSPaint illustration of this because I can't describe it.
Handstand pushups
Good fo yo delts.
Hindu Squats
I'm sure you know these.
Jumping Hindu Squats
More explosiveness = more fun (and more DOMS)
One Legged Squats (pistols)
No explanation needed.
Pullups
Yup.
Alternating Pullups
Apply the same concept from "Alternating Pushups" to your pullups.
One armed pullups
Pretty tough without a pullups bar.
Crunches
Good for teh abz.
Horse Stance
If you're a kung fu nut. Develops mad rooting, yo.
Ok, without the aid of a pullup bar, it's hard to work your lats/biceps without weights. It's also hard to work your calves without weights, and your hamstrings, too.
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