A good choice is biking. It is very kind to the joints. It is virtually non-impact in nature.
I am bone-on-bone, and ride daily. I save my impact for my ref schedule. I ride as much as possible on my regular bike. When it gets cold and wet, I choose a regular stationary bike at the Fitness Center.
You need to ride properly to avoid the 'medical' dangers of riding incorrectly. You need to learn to spin at 70+ rpm, with MODERATE pedal pressure. Spin, or ride, for a one hour.
The only time I feel pain is when riding, is when I am going up a steep hill. This is due to the high pedal pressure... kind of like raising the incline to maximum on a treadmill.
You do not have to give up the out of doors, or the fun, get a bike and ride outside. Use the stationary bike for the late evenings. IF you want this for weight loss try an elliptical rather than the trainer .... AND, make sure you use either of these with a good diet.
So, either trainer is o.k., but a recumbent is best for a bad back. Second, use a bike and enjoy the exercise. Diet. But most important, learn how to ride properly... read up, or ask on this forum, about 'spinning' and shifting.