I have two knee replacements and cannot run well outside. I am ok on treadmill but get bored. Used to do marathons and need the cardio to complement the weight training.
Swimming is maybe the best cardio sport but many don't have easy access to a pool.
I started cycling 6 years ago after starting to feel father time creeping up on me and it has done wonders for my cardio and overall fitness. I've never been really out of shape but cycling has made my cardio better now at age 49 than I was in my 20's. As a bonus biking has made the start of the ski season much easier - it no longer takes me 5 or 6 days of skiing to get my legs back.
I had minor knee surgery in 2011 on my left knee and a full ACL replacement in April of 2012. Cycling is one of the main reasons I recovered fully and quickly from my ACL surgery. I was able to do a 62 mile ride 6 weeks after surgery and a very hilly 100 mile ride 3 months afterwards. Two of my buddies made the switch from running to cycling and have had similar positive results. Good luck.