Let's finish up the lunge movement. When bodyweight lunges no longer challenge your skill and strength level, it's time to add an external resistance.
This can easily be achieve by holding a pair of dumbbells at your side, holding a kettlebell at chest height, or resting a barbell on the upper traps (back squat position) all while performing any lunge variation.
Believe me, the external load will be a necessary stimulus to spark new growth.
So, here we have it. Though not all inclusive, this discussion should give you a great start by incorporating lunges into your training routine. Just progress wisely and safely, and lunges can support your daily activity.
Make it a great day!