Finished with your squats? Good. Now do some lunges... and use a barbell.
Why? Lunges add total quad demolition and single-leg work. Grabbing a couple dumbbells will work, but barbell lunges avoid grip and posture fatigue while allowing greater loading.
You don't have to have an upright torso for these despite what many personal trainers still say. Instead, use a torso angle that best allows you to keep a neutral spine and engaged core while maximizing range of motion and avoiding knee discomfort. Too often, staying upright can only be done at the expense of an aggressive lumbar arch.
Lunges can be done with a vertical shin for more glute emphasis or with a forward knee for more quad focus, though all forms of lunges hammer quads. Emphasize the quad-dominant version by allowing your knee to travel as far past the toe as your ankle mobility will allow and your knee will tolerate (free of discomfort).
Maintain firm heel contact with the ground on each stride. Pissing off your knees will shut down your quad training, so use the form combining the best quad emphasis with knee happiness. Stop touching your back knee to the ground. The extra little range of motion isn't worth the impact on your kneecap.
Alternate forward strides across a room or lunge in place if you don't have the space to lunge across a floor. Do 3-4 sets to failure at about 10-15 reps per leg.
Take each stride forward instead of crossing your feet over as if walking a tightrope. This only makes balance harder with no training benefit.
Don't have the space to do these at your gym? Swap them for Bulgarian split squats.