Lots of people suggest swimming as stress-free on the knees. It's impact-free in any case, but note that the breaststroke kick can cause trouble. The "whip kick", which features a rotation of the knee (thighs and knees still fairly close together, but your lower legs pointing to the side with your feet reasonably far apart) doesn't agree with everyone.
Keep the breaststroke distance relatively short and swim it at a leisurely pace if you're susceptible to knee pain. Swim your more vigorous bouts freestyle/front crawl, backstroke & occasionally fly if you can.
To make the swimming less boring, swim different strokes, and pick up a few training schedules with different workouts to avoid getting into a rut with continuous swimming at constant speed - sets of endurance swimming, some intervals, some sprints, some technique drills and so on.
Frankly, even that gets boring if you do it on your own. Teaming up with others who swim at rougly the same level as you do is the best way to enjoy swimming.