I have invented a new abs workout that I have named "4 steps to greatness." It is performed as follows: Position yourself for plank, cheap i.e. forearms on ground, body flat and stiff like a plank, toes touching ground. All of the below steps are performed with only forearms and toes touching the ground at any time; back and legs are kept as straight as possible (no keeping butt high in the air!).
Step 1: Pick one foot off ground and place it down further away from where you started. (for right foot; move it to the right 1-2 feet).
Step 2: Pick up other foot and move it to the other side. (i.e. with right foot out to your right still, pick up left foot and move it 1-2 feet to the left).
Step 3: Pick up first foot and bring back to starting position. (right foot back to center)
Step 4: pick up second foot and bring back to starting position. (left foot back adjacent to right foot)
That is one rep. For me, 20 reps is enough to break a serious sweat and get my breathing going. The rhythm you get into makes it easy to continue on even though every muscle on the front of your body is starting to burn. This will work out your core, hip flexors, and quads pretty well. I feel it most in my abs, my roommate feels it most in his quads (guess he has weak legs?). If the exercise is too difficult, start by moving feet outwards a short distance; if the exercise is too hard, move feet as far apart as possible (and do it SLOW, then we'll see how tough you are).
I can already think of variations, where instead of resting on your forearms, you hold a half-pushup position...it's basically doing one-arm pushups at that point. Can also move hands far out to the sides for generating ever more torque on your core.