You can do knee tuck push-ups without a stability ball too, just (obviously) one knee at a time, bringing the knee across and in toward the opposite shoulder. Or push up with your feet on the ground and your hands on the ball.

"Gliding" is popular these days too, you can do it on carpet with paper plates or on hardwood with fleece rags (fuzzy side down). Any of the push-up variations that we used to do on the slideboards, and more. Put the plates under your hands and bring your arms wide on the down phase, back under your shoulders when you push up. Start on your knees and extend your arms overhead, then back under your shoulders to push up, kind of like those ab wheel exercises. Then you can put the plates under your toes and do knee tuck push-ups without raising the leg, just sliding it along the floor (which makes you keep your hips down farther), or the same thing with a hip abduction.