If you can manage to remember to do it, try this. Walk on your tip-toes as much as possible. Usually I only do it at night around the house. If I remember I do it in the morning. Try to make it a habit, obviously don't go crazy with it. Just when you're at home alone and you're doing something, just try to do it.
Calves are like forearms, they are accustomed to continuous use. You have to really push them. Problem is, you can't really do any huge weights required to grow them like other muscles. And a herniated disc further complicates things, I know, I had surgery for one 3 months ago.
Besides the tip-toe deal, I do standing calf raises now, bodyweight only. Like you, I don't lift heavy weights that compress the spine. I like to do alot of explosive partials too, from the floor. Hold the flexed position for a couple of seconds. There are ways to overload muscles besides just bluntly lifting heavy weight. I don't go to the gym, but if I did, I'd do the seated calve raise machine also to get some weight training involved.
Obviously there are other ways, but this is for a guy with a disc injury.