More or less in order from easiest to hardest:
Planks
- Can be done resting on hands or forearms/elbows, knees or toes, depending on comfort and level of difficulty.
- To work on obliques, shoulders, and balance, combine with alternating lifting an arm parallel to the ground (do the opposing leg at the same time for even more challenge)
Crunches
- Can be done with feet on the ground, knees at right angles with lower legs parallel to the ground, or legs straight up in the air, depending on level of difficulty desired.
- Do straight and oblique (left elbow to right knee, then right elbow to left knee)
Sit-ups
- Can increase difficulty and work shoulders by holding a weight (medicine ball, dumbbell, small child, etc.) out in front of you (while lying down), and transitioning it to directly overhead as you sit up.
- May not be good for your lower back if you currently have a weak core.
If you have a medicine ball, or other non-breakable weight (don't use the small child for this one):
Sit on the floor, with your torso leaned back to about 45 degrees, back straight; legs can either be bent with feet on the floor, or held out straight (balancing on your butt). Start with the weight to one side, and lift it over your lap all the way to the ground on the other side. Can be made harder and work shoulders by going over your head instead of in front of you.
Spider walks:
Start in a plank position (must be on hands and toes for this one). Walk yourself to the side (move right hand and left leg at the same time, and vice versa). When you run out of space, go back the other way.
If you have a HEAVY piece of furniture, or something else fixed to the ground that you can grab:
Lie on your back with the object just at the top of your head. Grab it, and use it to keep your upper back on the ground. Keeping your legs straight, slowly lift them perpendicular to the floor, then slowly lower them back down. To make it harder, when your legs are vertical, use your abs to lift your legs and pelvis off the ground before you lower your legs.
Yes, I spend way too much time at the gym...