What are good exercises to relieve pain from lumbar spinal stenosis?

Not that simple.: Physical therapy for lumbar spinal stenosis may include modalities, stretching, strengthening, conditioning. Rather than just giving a set of exercises, this is more of a "process" that is best addressed by hands-on from the therapist. It will take some time and follow-up to address this adequately and make improvement. I would not suggest just saying that certain exercises are good for this.

Flexion: Most exercises help but ones with your lower back in fkexion such as biking or walikng uphill or on a incline are good as are williams flexion exercises and swimming pool workouts. Core exercises are helpful like pilates.