Scoliosis surgery is spinal fusion - several segments of the spine have been fused together with bone. This means the spine doesn't bend at all in that area. This limits somewhat what one can do for exercise. Anything that primarily involves spine motion isn't a good idea - crunches, for example. But "back" workout is OK, like lat pulls, pullups, rows. Chest, triceps, biceps, legs can all be exercised safely. Running probably isn't such a good idea, and cycling may be difficult due to leaning over.
This all applies if there is no pain or discomfort. If you have back pain with any activity, you shouldn't do it. This ISN'T a "no pain, no gain" situation.
In this situation, maybe using an elliptical cross-trainer for cardio would be a good idea, especially if you're going to a gym that has them anyway. The lack of impact is great for joints, and it allows you to stay upright unlike biking.
Not sure what kind of ab work could be done without involving spine motion... might need to check with your doctor about that. But doing a lot of things standing (like standing cable rows vs. seated) will give your core some stability work to do, and should be perfectly fine, since the purpose is keeping your spine still instead of flexing it.
