certainly sounds like you are looking more to get big than get strong. Anyways, obviously you want to find compound movements:
Hack squats (I have one in my gym where the weight rests on your glutes so that wouldn't put stress on your L5 I would think)
Glute ham raises
Other than that the choices are limited as far as I can think. Leg extensions and curls are really garbage exercises in my opinion but if thats all you can do, then push it hard! As far as calves... Well I haven't trained calves in probably 9 months soooo... calve raises? Hahaha anyways. I had an injury preventing me from sq/dl a while ago and I just pushed as hard as I could on everything I could do (mostly curls/extensions) and I managed to not suffer too much.