What exercise is good to trim waistline after lower back surgery?

Cardio. For weight loss you'll need to do something that burns calories. Water aerobics classes are run by the arthritis foundation. Stationary bike, elliptical, walking. A lot depends on how long ago was the back surgery. You might consider a few sessions of physical therapy prescribed to make an exercise plan together.
Core strengthening. In addtion to healthy diet, walking, core strengthening exercises (under supervision initially) should help get your back healthier, and trim your waistline in the process.