Besides push-ups, pull-ups and sit-ups are there any other exercises using your own body weight?
standing ab roll out
do sit ups with your legs wrapped around a heavy bag
do sit ups on a bench with 2 pairs of foam cylinder pads. get your shins under the pair of foam pads farther away from you and your hamstrings on top of the pair closer to you and you do sit ups without your entire bodying anything other than you legs touching the pads
a variation of hanging leg raise that requires you to start at a dead hang on a pull up bar, while doing half a pull up you also do a knee raise at the same time. Then you shift yourself back so that your head is under the level of your butt from the side view, and you extend your legs all the way out and the position is almost like you're doing a dragon flag, then you reverse the all the motion, and keep repeating.
find a pole and hold yourself upright horizontally