Stand up tall and take a big step froward so that when you bend your knees your front knee stays above the ankle. Try to get deep enough in the lunge position that your back knee touches the floor, or comes close. Then step the back leg to meet the front and continue forward.
This movement combines a normal crunch with a reverse crunch to give you a more full range of motion and more difficult exercise. Start on your back in a crunch position with your feet off the ground, knees bent to 90 degrees. Take a deep breath in and crunch in tight to create as minimal space between your knees and elbows as you can. Slowly exhale (to help keep your core engaged) while lowering your feet toward the floor and your upper back and head toward the floor at the same time. Crunch back while bringing your feet back to the starting position, and repeat!