Sadly, few things compare to the squat for building mass and power. And, with disc issues (if they haven't been resolved via decompression or other means), you'll definitely want to avoid any leg pressing as that's the surest way to cause undue stress on affected vertebrae.
To ask, do you have access to a reverse hyperextension machine? With proper use of one and some good rehab, you may be able to squat and deadlift again one day (lots of guys out there in powerlifting who were told to never train again after similar injuries who came back stronger than ever), but they're not always easy to find. I swear by the decompression treatments I had done as the only reason I can squat and deadlift again, but they're expensive and also not always easy to come by.
First thing I'd recommend is a belt squat setup like the one from IronMind. I have one, bought it when my back was really bad, it did allow me to squat light weight for high reps with zero disc pain or stress, but it takes a bit to get used to. Here's a link to one:http://www.ironmind-store.com/SUPER-SQUATS174-Hip-Belt/productinfo/1220
If you have to make do without anything extra, I'd suggest lots of walking lunges with dumbbells in your hands to keep from any added back strain, perhaps some plyometrics for explosive power, and the usual old leg extensions and hamstring curls. If you have to avoid compression then there's not a whole lot more than some of the mediocre things that exist that won't exacerbate pre-existing injuries, but something is better than nothing.
The hip belt squats with something like the IronMind product would allow you to definitely work hard enough to build some good size and strength back, probably what I'd recommend most for the time being. And, of course, if you can find a good reverse hyperextension machine to use (must be a good one with a natural ROM, not a poor knock-off that feels unnatural), or, if you can try a long round of decompression treatments (at least 20 sessions, 30 is preferable), you might find those things to be life-changing as well for helping your back issues to practically be gone over time.
Wish I had more recommendations for you, just figured I'd share what I've learned these past few years with my own L5-L7 issues!