Knee (meniscal tear) pain and exercise concerns

A few years ago, I was climbing the stairs in the subway when suddenly my right knee hurt so badly that I barely managed to drag my leg home. The pain was so severe that I had an MRI the next day, which revealed a meniscal tear. Even with surgical suturing, there was a high risk of re-tear, so the doctor did not recommend surgery. Just in case, I saw two doctors, and they both had the same opinion. I was told that strengthening the muscles around the knee would reduce the pain, so I started riding an indoor bicycle, and the pain decreased.

However, these days indoor cycling is too boring and dull. What kind of exercises can help strengthen the muscles around the knees?

Are there any exercises suitable for women in their 40s that are not too strenuous?