Here's one I like to do in my classes. They beg for 10,000 crunches rather than this series!
Low plank position. Elbows bent, forearms on floor, elbows lined up right under your shoulders so your upper arms are perpendicular to the floor. Make a straight line from your head to your heels. Don't let your belly button drop toward the floor. If you feel pressure in your lower back, you may need to lift your hips a teeny, tiny bit up toward the ceiling. Breathe in through your nose, out through your mouth. When you inhale, think about filling your lungs with air and expanding them left to right and front to back. When you exhale, force all the air out of your mouth and pull your belly button in toward your spine and up toward your ribs. Hold the plank for 1 minute.
After a (very) brief rest, get into that same low plank position. Hold for about 30 seconds, breathing as listed above. Then start to add movement. Without losing your plank position,
slowly drop one knee toward the floor, then straighten the leg. Then do the other knee. Keep alternating, very slowly, for another 30-40 seconds. Your goal is to keep your plank intact. The only thing moving should be your knees, slowly down toward the floor then back up.
After another (very) brief rest, get back into that plank position again. This time, instead of moving the legs, you are going to move the hips side to side. Once again without losing your plank position,
slowly twist your entire lower body to lower one hip toward the floor, then back up to center. Then the other hip toward the floor. The twist should originate from the top of the rib cage. I find it easier to do this without shoes. When we do it in class, I have them move their feet close together (touching). As they drop their left hip toward the floor, they roll onto the outside of their left foot. Same on the right side. Do about 20-30 twists, total. SLOW. Keep that plank nice and strong. Don't let your shoulders start to creep up toward your ears.
Ahhhh yeah. So evil.