A great body weight exercise you can do anywhere which really works the core is called a Plank.
To do a plank you want to get on the floor and have your forearms resting on the floor and you want to get up on your toes. Be sure to keep your body in a straight line from your head to your heels making sure that your rear end isn’t sticking up in the air and that your mid section isn’t going towards the floor. You want to hold the position for 20 – 60 seconds. Lower and relax your body for 30-60 seconds and repeat for 3-5 reps.