For hip flexors, marching in place with ankle weights on (if you have some) works if you bring your working leg up nice and high. Also, with or without ankle weights, seated on floor with legs straight in front of you, back straight, lift and lower leg as many times as you can. I had to do a lot of hip flexor rehab after fracturing my pelvis as I was much, much weaker on the right side, and these are the ones my PT had me do. Still challenging if you do enough of them!