Can't decide if the door should be open or closed? Why don't you hold on and decide after you crank out a few squats. Squats help your legs and hips get stronger and are a great strengthening exercise. Using the door to hold on to will help you keep yourself more upright and get your weight back further to engage your hips more.
Start in standing while holding onto the door knob on both sides. Shift your weight onto your heels, bend your knees and bring your butt down to squat. Use the door handles to hold onto so you can shift your weight back and keep yourself upright with your knees behind your toes. Come back up and repeat until you can't do anymore. Or you need to shut the door. Whichever comes first.