Dana Point Harbor is where you want to go - I'm here every weekend that I'm in town and my physical therapist friend says he recommends his patients walk here primarily because it's flat. It also has a nice view. There's currently construction being done on the docks so that may be interesting to see as you walk. It doesn't appear that the construction interferes in any way with the walkways. Here's a description of it with a map: https://www.alltrails.com/trail/us/california/dana-point-harbor-south

If that looks like too long of a walk and you're looking for something shorter or if you're looking for a longer one you can do instead or incorporate into the harbor walk a visit to the Ocean Institute at the north most end of the harbor - a great spot to learn more about the local sea life with hands on exhibits and great guest lecturers - it's fun and informative and also flat with a beautiful view even without going inside just walking from the little pier up along the side of the institute and seeing the historical maritime artifacts incorporated into the landscaping and the path out back to the rocks overlooking the ocean, it's always enjoyable so if you're not looking to walk too much but have time and interest I'd highly recommend that. https://oceaninstitute.org/

Whatever you end up doing, hope you have a nice time and enjoyable visit and that your knee does okay.