If all you can do is walk, then walk, and walk some more. Cardio and other exercise aren't very effective for fat loss, so you don't really gain anything as far as helping lose weight goes, but there are many other health benefits, of course. Most of the benefits can come from regular walking and other activities. I didn't do much pre-surgery, but post-surgery I started walking daily and got up to several miles a day within a month post-op.
Doing some resistance exercise (working muscles) is definitely recommended after surgery while the rapid weight loss is happening to minimize muscle loss compared to fat loss, but it doesn't take too much time and it doesn't have to be that intense. You can just use your existing body weight. No need for weights and such, but things like resistance bands can add a bit to it. Obviously, it still requires a bit of commitment and motivation, but anything can be turned into a habit. The hard part is getting started and doing things long enough to reach that point of it being normal.
A few months after surgery, I decided to get physical therapy to help get more comfortable with exercise movements since my body had changed so much. It was really helpful and sort of forced me to do more than I had been.
Just being active and moving is enough to make a difference. It doesn't have to be an "exercise" plan per se. It could be gardening or some other outdoor hobby, hikes, bowling, golf, housework, hobbies, etc. Push yourself to go out and learn some new things that involve movement and fun, etc.