Here is a fast circuit workout for your core. Set your timer for 30 seconds. You'll do each move for 30 seconds, rest for 30 seconds then repeat all three exercises two more times.
Here is a basic explanation of each move:
Scissors - Lie flat on your back. You can extend your arms along the sides of your body with your palms pressing into the floor, or you can bend your elbows and place your palms under the back of your head. Pull your navel in towards your spine (engaging your abs) and actively press your lower back flat on the ground. Lift both legs straight up toward the ceiling, continuing to engage your abs and press your lower back into the ground throughout the exercise. Slowly lower your right leg down toward the ground, until it is a few inches above the floor. Then scissor your legs, so you lift your right leg back up as you lower your left leg down towards the ground. Keep this up with slow and controlled movements.
Russian Twist - Sit with your knees bent and heels on the floor. Engage your abs and lean back until your upper body is at a 45 degree angle to the floor, keeping your back long and flat. Rotate your torso, reaching both hands toward the floor on the side before returning back to center then reaching for the other side. A weight or medicine ball can be held and/or feet lifted off the ground to make this more challenging.
Plank - Lie facedown with legs extended and elbows bent and directly under shoulders; place your hands flat. Feet should be hip-width apart, and elbows should be shoulder-width apart. Contract your abs, then tuck your toes to lift your body (forearms remain on the ground); you should be in a straight line from head to heels. Hold for 30 seconds.
Do this two to three times a week on non-consecutive days for stronger abs!