Since you have back issues you might want to try frontal kicks. It sounds easy and it is, but is a good way to train the quads with no weights.
Get a stool about 14 inches tall, you might want a dowel or broom or something to help you keep balanced. Stand behind the stool, place one foot on top of the stool, and in the opposite hand have your balance stick if needed, then, simply kick the leg forward with the foot flexed, but when your foot comes down to the stool, try not to touch it, or if you do, touch with just the toes very lightly and very briefly. There should be no momentum guiding these kicks, they are quick, but steady.
Now sit on top of the box, and place a dumbbell on your quad (if you want one, this can be done without as well) flex one foot, place the other on the floor, legs together. Now simply lift the leg, then press the leg out fully without touching the floor, then bring the knee back up with the quad muscle.
1, 2 an excercise I'll do
3, 4 get down on the floor
5, 6 my shape I'll fix
7, 8 I hate this weight
9, 10 can I do it again?