The general consensus from trainers who actually train people with Spondylolithesis is that flexion/extension exercises of the spine such as sit-ups and back extensions tend to aggravate the symptoms while exercises that hold the spine in normal anatomical position isometrically, like squats and deadlifts, are not only fine but necessary for stabilizing the injury. Anything overhead will have to be done carefully, but are still valuable. Form will have to be very good and I wouldn't recommend trying to teach yourself through trial and error, a good coach here is indispensable.
The doctors that recommend the complete cessation of heavy lifting in cases like these are the ones who don't fully understand the bio-mechanics of the structures involved.
If you are one of the majority of Americans whom have never squatted or deadlifted, increasing strength would likely be a good place to start, but start slow.