Side knee to elbow plank: In a plank position (elbows harder, hands easier) bring one knee to the outside of your body as close to the same side elbow as possible. Return the leg so you are in a plank position again, and quickly do the same with the opposite side, alternating in this fashion for 15 reps per side or 30 seconds for 4 sets. Take a 15 second rest between sets. Make sure to keep the butt down and focus on getting a good side bend as you bring the knees up.
Double leg plank hop: From a plank position on your hands, jump both legs in towards your chest, keeping your weight on your hands. Immediately jump back to the starting position. Do this for 15 repetitions or 30 seconds for 4 sets, resting 15 seconds between each set.
Cross climbers: Set up as if you were going to do mountain climbers; in a plank position on your hands. Without letting the hips turn too much, cross one leg under your body towards the opposite side elbow, squeezing the abs to dictate how far you can go. Return to the starting position, and change legs. Do 15 reps per side or 30 seconds for 4 sets, resting 15 seconds in between.
— Chase, Northern Illinois University