The towel is your friend...
And to prove it to you, I've got 10 ways you can use your gym towel to get a great workout (not counting mopping up sweat, of course).
1. Towel Chin-Ups
Throw a towel over the chin-up bar and grip the ends. Now pull yourself up! Gripping and holding your entire bodyweight up on a loose towel is GREAT for the grip and will activate a lot of muscle fibers in your lats because of the instability.
2. Towel Calf Stretch
Lay on your back on the floor and loop your towel around the forefoot area of one of your shoes. Now pull HARD towards you, stretching the calf. This technique works extremely well because you can very easily adjust the tension of the stretch. Forget fancy stretch straps - just throw a towel over!
3. Abdominal Sit-Ups
This is a GREAT abdominal exercise...roll up a towel, lay down on the floor and slide it under your lower back. Now do a sit-up. The rolled-up towel braces your lower back so that instead of using hip flexors and putting stress on the lower back, all the tension of the exercise goes directly onto your ABS.
Be sure your not pulling on your head - you can also make it easier by holding your arms across your chest or at your sides.
4. Towel Bodyweight Tricep Extensions
Throw a towel over a railing or something else a few feet off the ground. Now hold onto the ends. Set your feet back a few feet. Now, keeping your body stiff, bend at the elbows ONLY then extend back up. The instability of the towel grip will set your triceps on fire!
These pics show the exercise done on a bar - just loop a towel over the bar and grip the ends. Move your feet closer to make the exercise easier.
5. Towel "Fat" Bar/Handles
Thick bar and thick handle training is GREAT for the forearms and grip strength. You can accomplish this cheaply by simply wrapping a gym around the bar or the dumbell handles and then gripping around THAT as well.
The padding and "squeezability" (I sound like a toilet paper commercial...cripes) of the towel around the bar activates a tremendous amount of muscle fibers in the forearms and hands.
6. Towel Pushdowns and/or Pulldowns
The easiest way to set this up is to attach a single handle to a high pulley then loop a towel through that. Grip on the ends of the towel and do a pushdown or pulldown (depending, of course, on what exercise you want to do)! It's just like using a rope attachment but without the solidness of a rope (or the knots on the end to brace your hands).
7. Towel Pull-Up Rows
This exercise gives you some excellent options for ...