Plank: Start in a push-up position with your arms straight and hands shoulder-width apart. Lower your forearms to the ground, keeping your elbows directly under your shoulders. Engage your core and hold the position for 30-60 seconds.
Bicycle crunches: Lie on your back with your knees bent and your hands behind your head. Lift your head, shoulders, and feet off the ground. Bring your left elbow to your right knee while straightening your left leg. Repeat on the other side, and continue alternating sides for 12-15 reps.
Russian twists: Sit on the ground with your knees bent and feet flat on the floor. Lean back slightly and lift your feet off the ground. Hold a weight or medicine ball with both hands and rotate your torso from side to side, tapping the weight on the ground each time. Do 12-15 reps.
Leg raises: Lie on your back with your legs straight and your hands at your sides. Lift your legs straight up to the ceiling, engaging your core. Slowly lower them back down to the ground, without touching the floor. Do 12-15 reps.
Mountain climbers: Start in a plank position. Bring your right knee up towards your chest, then quickly switch legs, bringing your left knee up towards your chest. Keep switching legs as fast as you can for 30-60 seconds.