Put both of your hands together and do a push up. If you have someone that can help you, you can do these. Lay on your stomach and hold the towel behind your neck in the middle. Now have your partner try to pull the towel down to your feet. Pull away from the feet and try to work the arms.
Donkey kick back with towel.
Tie the towel to a door or something that will not get broken if the towel is pulled. Now grab the one end of the towel. Lean back a little away from the wall to give the towel resistance. Now try to straighten out the arm, at the same time you have to move your body to work the resistance. Oposite resistance curls.