Day four: Inchworm
A silly name but 'inching' is the name of the game. Standing with feet about hip width apart, lean forward to reach the floor with your hands. Inch forward so you end up in a press up position (feel free to do a press up at this point if you wish!), then inch your way back and lift off the ground until you're back to standing. A great whole-body exercise. If your hamstrings are tight and you can't get down to the ground without bending your legs (or falling!), then have your legs wider.
And, not forgetting your reverse crunch, press ups and plank