What kind of exercise is good for a herniated disc?

McKenzie. Talk to your therapist about mckenzie directional-preference based therapy. In most people excercises of lumbar extension help to centralize pain, but not the case in all people with disc herniations. Far lateral disc herniations tend to respond to lumbar flexion exercises better. Get formal evaluation to see what directional preference therapy best for you.
Careful exercise. Start by avoiding prolonged positioning & repetitive twist or bend maneuvers. If you had physical therapy, resume those exercises. Use of otc medication will help control symptoms. Cardiovascular workouts may help except for rowing & maybe running. If you did not have pt, you may want to get a session to learn exercises. Weight control & not smoking are key. Time is biggest healer.
Core muscle strength. You need to do abdominal and low back strengthening and flexibility training. There are certain exercise regimens that are designed to stabilize the core muscles in the trunk to help with herniated disc issues.