Towel Pullups may look silly at first glance, but they are extremely effective and is often used by arm wrestlers and rock climbers.
If you find regular pull-ups to be easy, you will be humbled because the towel pull-up is much harder!
The towel pull-up will not only challenge you back but literally every muscle in the upper body including your chest to stabilize and your forearms and the smaller muscles in the hand to grip the bar.