Here are 4 of my favourite core exercises that will spice up a plank and can be done anywhere. Add these in at the end of your workout or intersperse each one throughout your strength training routine.

Superman Plank
This is a favourite of mine.  Start in a plank position then extend out one arm straight while making a fist and hold yourself still, ensuring that your hips are parallel to the ground.  Start with 10 seconds a side then work up to 30 seconds or more.

Side plank with rotation
This is a great exercise to work your waist line. Begin in a side plank with your feet staggered on the floor, placing your top leg ahead of your bottom leg. Start with your top arm up in the air and rotate through your hips and your feet as you bring your top arm underneath your body. Continue to rotate until you are in a single arm front plank position and then return your top arm back up into the air. Start with 5 and work up to sets of 10 or 20 a side.

Side Plank with a knee and elbow touch
Begin in a side plank with your feet stacked and your body weight directly on top of your elbow and your top arm stretched out over your head.  Without allowing your hip to drop bring your top knee and your top elbow towards each other without letting your torso flex (use your core muscles to keep your body as still as you can).  Return back to the starting position and repeat.  Start with 10 and work up to 20.  If this is too hard for you then you can bend your bottom leg and place your knee on the ground rather than your foot.

Front plank up/downs
This core exercise works the abdominal muscles while at the same time strengthening your arms, shoulders, chest and back.  Start in a plank position and while keeping your hips from rocking side to side you will place your left hand on the floor and straighten your left arm.  Do the same on the right and then lower yourself back down into the plank by placing your left elbow back onto the floor and then the right.  Do 10 on the left side then 10 on the right and work up to 20 or more a side.