Here is a great exercise that very few people utilize. While every one is off wasting their time doing crunches and sit ups focus on this exercise to have a 6 pack come beach season.
The mountain climber is a great body exercise that will work your core, legs, arms and shoulders. With many variations this exercise won't get old anytime soon.
1. Start in a push up position.
2. Brace your core.
3. Slowly bring your right knee up to your right elbow.
4. Lower and repeat with the other knee. That is one one rep.
The only time your toe should be touching the floor is at the start and finish. This will maximize core effort, help stabilize the lower back all while helping flexibility and reducing back pain. Your arms and shoulders will also get a work out from the stabilizing they will be doing during the exericse not to mention your quads.
For an easy variation do the mountain climb twist. Instead of bringing your right knee to your right elbow, cross the right knee over to the left elbow. Your shoulders should remain straight, no twisting should happen there. There are more difficult variations but these are the basics and should be mastered before moving on. Once you can do 30 twists on each side then its time to add ankle weights or new variations such as having your feet on a BOSU ball or hands on a medicine ball.