Planks: Facedown, hold your body in a straight line, resting on your elbows. 2 x 15-60 seconds.
Body-Weight Lunges: Keeping your torso upright, step forward and kneel until your front knee is at a 90-degree angle; repeat with your other leg. 2 x 12 reps.
Bicycle Abs: Lie on your back with knees in the air. Keeping lower legs nearly parallel to the floor, slowly pedal your feet, six to 12 inches off the ground. 2 x 12-15 reps.
Side Plank: Form a plank on one side, with weight on one elbow. 2 x 15-60 seconds. Switch sides.
Step-ups: Find a bench just above knee height. Keeping your front knee behind your front foot and an upright back, step slowly onto the bench. 2 x 6-12 reps.