Bulgarian Split Squats are an excellent single-leg exercise that allow you to target your legs without loading your spine in the same way as a back squat.
I like to use higher rep ranges when using this exercise (12-15) to get a good pump in the muscle. However, some people have issues with their rear foot during Bulgarian Split Squats off a bench (I.e pain in the ankle/foot, cramp or you just have issues with foot placement). If this is you, try elevating it on a foam roller, it might just eliminate the issue so that you can concentrate on the muscles that you are trying to target.
To progress the exercise, try holding a DB in one of your hands, or in both if you really want to up the load.