The Stepup is arguably the most functional lower body exercise. Consider it the hip-dominant sibling to the lunge (and it’s also easier on the knees). Unlike the lunge, it requires you to lift your leg up, recruiting the hip flexors more. The ideal starting step height is where your top thigh is approximately parallel to the floor (or your box squat height). You can gradually progress to a higher box over time but you should always be able to keep your back foot flat on the floor at the starting position.
You should be able to do 10 minutes of continuous alternating stepups before adding weight. To get the most out of this exercise, accentuate the eccentric phase by taking 3-5 seconds to step down. Explode up to maximize fast-twitch fiber activation.
Hold a pair of dumbbells for size & strength. Hold a dumbbell in either hand to increase core & hip activation. My favorite variation is the stepup to reverse lunge for the best of both worlds. 10-20 minutes of continuous alternating stepups is an amazingly mindless workout on days when stress is high & focus is fading.