I found this great core strengthen routine from Womenshealthmag.com. I tried the routine myself and felt the burn in my arms, abs, and legs. It's defiantly worth trying in your free time!
1. Opposite Knee Tuck

Directions: Start in plank with your feet hip-width apart and hands directly beneath your shoulders. Lift your left foot about 6 inches off the ground, then bend your left knee and bring it toward your right elbow. Hold for 3 seconds before returning to starting position. Repeat, bringing your right knee toward your left elbow.
Do: 10 sets, 1 reps
2. Oblique Grab

Directions: Start in plank with your feet hip-width apart. Lift your right arm, then wrap it under your body and grab your left side just below your ribs. Hold for 3 seconds and return to start. Repeat, grabbing the right oblique with your left arm.
Do: 10 sets, 1 reps
3. Side-Plank Flip

Directions: Start in plank with your feet together. Lift your right arm out to the side as you rotate your body 90 degrees so you're balancing on the outside of your left foot and your left hand. Hold for 3 counts and return to plank. Repeat on the left side.
Do: 10 sets, 1 reps
4. Two-Limb Plank

Directions: Start in plank with your feet hip-width apart. Keeping your neck in line with your spine, simultaneously raise your right arm straight ahead (so your bicep is next to your right ear) and lift your left leg 6 inches off the floor. Hold for 3 seconds, keeping your hips and shoulders perfectly level. Return to plank and repeat on the opposite side. That's 1 rep.
Do: 10 sets, 0 reps
SOURCE: womens health mag