Starting exercise after spinal fusion back surgery

Back story: I have had a back injury at my L5-S1 vertebrae for quite some time (pain for past 11 years) which has gotten progressively worse as time went on. I was in excellent shape (the best shape of my life 2 years ago) but had to back off the time and intensity about 1 1/2 years ago due to increased pain and time constraints. This past February I began getting leg tingliness which progressed to severe leg numbness. The past couple months my exercise was very limited due to this leg pain/numbness and back pain, and really couldn't do anything at all the few weeks leading up to my spinal fusion surgery on June 2.

I have been walking a lot (during my 4 weeks off work I was taking 3-4 daily walks, about 1.5-2 hours a day, 3.5-5.5 miles per day) but a little bit less now that I am back at work (usually one long walk in the evening, 45-60 minutes in the evening and walking the stairs at work periodically). Right now I have a 10 pound weight limit on lifting any free weight type thing (about the weight of a gallon of milk). At the 4 week check up the doctor gave me the OK to start doing more working out but I am still strictly prohibited from doing any twisting at all. I obviously can't/don't want to run at this point but I enjoy doing the stair machine at the gym, and I can't do the elliptical since the body tends to naturally twist slightly.
Any recommendations for weight lifting (dr. recommended starting with lighter weights than I think I can handle and doing more reps, and building up over the next couple of months) and other cardio/exercise that doesn't involve twisting?