Chin ups and pull ups are one of my most favorite exercises! They are unbelievable for the back and with the exception of a few other small exercises, they are all I did for back for almost a year. However, all that time just hanging around on the bar made me think of other exercises I could be doing to improve my physique. I came up with some ab exercises that I could do in between sets or even as stand alone reps on ab day.
The first is the simple knee raise. I just hang from the bar and bring my knees as close to my chest as I can get them. Then I slowly let them go back down to the floor. Although these feel easy at first, doing thirty or so will leave you feeling the pain tomorrow.
Another version is the straight leg kicks. Same thing of hanging from the bar with the arms straight. Then keeping your legs locked straight, raise them together as high as you can until you start to bend them. Slowly lower them back to the ground. These are more challenging than the knee raises as they add more resistance because of the change in weight displacement.
Knee up and twists are also great. To do these bring your knees up towards your chest again and then twist them sideways at the last second. Keep both knees together and try to make a parallel line with your lower legs and the floor. If paused in the up position, the bottom of your feet should be facing a wall and your knees pointing to the opposite one. Make sure to alternate directions with each rep.
Running in place is an odd one to do but works well. Hang from the bar and then run in place. Instead of doing normal steps, exaggerate each step by bringing the knee into the air as high as you can get it for each rep.
You can ride the bike too. While hanging on the bar bring your legs parallel to the floor as if you are now the letter L. Then start riding your imaginary bike for as many reps as you can.
Then there is the gorilla cannon ball. This is a fun exercise that works so many muscles. Hang from the bar as usual then bring your knees up to your chest as you are pulling your entire body towards the bar. Basically you are doing a chin up and trying to get your knees to touch the bar. It looks almost like you are about to do a cannon ball into the pool and at the same time you will feel like a muscle bound gorilla once you get off of the bar.