Partner training is great for lots of things—from helping you lift more to improving your morale—but training legs with a spotter is especially helpful. Take leg extensions, for example. You can devote your sole focus to squeezing at the top of the lift and keeping your quads tight.
You can do anything for 3-5 reps! Keep your movements fluid, maintain a controlled motion, slow down on the eccentric portion, and drive through the lift.