Why make it complicated? Crunches is an excellent go-to exercise and if you want to spice them up you can make them twisted.
Lay on your back in bed and elevate your legs. Raise your upper body and turn to the left and meet your left knee with your elbow or hand. Keep you legs elevate and switch sides. Do as many as you can manage.
There’s no need to raise your upper body all that much. If you just get your shoulder off the sheets then that’s enough.
HOW: You can do this exercise both as a static or dynamic workout. Most effective is the dynamic motion which requires both strength and coordination; Lean forward on a chair with wheels and assume the plank position, lower arms resting firmly on the cushion and upper arms in vertical position. Let the chair roll forward away from you as far as you can manage and then pull it back towards you again. Repeat! It’s a very effective workout and you will instantly feel your abs getting a full rundown.
It’s easier as a static workout. Then you just lean forward in a plank position and hold for as long as you can. You can also do this on a chair without wheels.