Ryan Hoover: In my opinion, one of the simplest but toughest exercises is the 100-Yard Power Wheel Challenge, which we regularly use at Fit to Fight. The University of California at Berkeley did a study that deemed the Power Wheel "the most effective core trainer." My clients and students think it's evil.
How To: Assume a Push-Up position with your feet on a power wheel.
Crawl for 100 yards.
Rest as needed, but know your result will be timed.