The forwards and backwards motion of this crawling move will work your abs like crazy, along with your shoulders, legs, and butt.
How to do it: Start on all fours, brace your abs in tight, and lift your hips slightly to raise knees off the floor. Step forward with right hand and right foot, and then immediately do the same with the left side. Build up your speed until you are crawling forward quickly (taking about 10 total 'steps') and then reverse your movement, stepping backwards with the right hand and foot, followed by the left, for 10 steps. That's one set. Try 3 sets total, taking a brief rest in between sets.