So, in honor of thumbing my nose at the certifying organization to which I worked like a dog to become certified by, I'm starting this thread for all of you to post your best exercises using non-standard gym equipment or any creative uses you've come up with for standard gym equipment.
If I think of more, I will post them, but these come to mind at the moment:
I use a folded up towel for stability exercises. Fold up a bath towel is as many directions as you can and then stand on it with one leg and do functional training exercises (forward reach) or upper body work.
I have a small children's ball that has gotten squishy over time and I use that a lot. I have clients hold it for a focal point during oblique twists. Sometimes I'll have them position it in the back of the knee (between ham and calf) for a glute press that's done on all 4's. It's a good starting point for a game of catch when they are unbalanced.