A gymnastics staple exercise, hollow body holds may look simple but they're a killer core exercise.
Lie down flat on your back and contract your abs, pulling your belly button toward the floor. The arms and legs should be held straight out from the body with hands and toes pointed. Slowly raise your shoulders and legs from the ground as you keep your lower back in contact with the floor. The goal is to find the lowest position that you can hold the arms and legs, without them touching the floor and without breaking lower-back contact.
Make sure you're pressing your belly button toward the floor and engaging your pelvic muscles for maximum effect.
Beginner variation: Make this exercise slightly easier by reaching your arms toward your feet. You can also try bending your knees instead.