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.
Situation: Sitting down, while at work
Sit on the edge of the chair and lean forward and grab hold of your left foot with your righ hand. Without arching your back, bring your belly towards your thighs. You should feel the tension on the back of your left leg. Keep the stretch for about 10 seconds and repeat a couple of times before switching leg.
Try to connect an exercise to something you do frequently, for instance picking out ingredients for when you are cooking. Try to do 5 squats for every new ingredient you pull out. It’s a piece of cake but some dishes have a lot of ingredients and the more you cook the fitter you’ll get.