If you are wanting to really work your core, I recommend using a Bosu ball. Most gyms have these flat balls scattered throughout the gym.
Here are some common workouts to try:
- Push ups with the ball side down (you are working your balance and your core)
- Sit ups/crunches with the ball side up (easier on your back than on floor)
- Squats with the ball side up (again, stabilization of your core)
- Stand and Balance with the ball side up while doing any type of weight training (bicep curls, tricep kickbacks)
And of course, it is now made in pink which is a new reason to get a ball of your own.