Place 2 kettlebells bottoms up in front of you. Place your palms on the bottom of the bells. I use small KBs for better balance. Then simply crank out push ups. This is challenging because the little devils are not very co-operative. Needless to say you have to stay pretty tight in your abs and glutes. This is even more challenging then doing push ups on the swiss or med ball or even gymnastic rings. Great for core stabilization. Give it a try.