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

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.
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.