As much as I enjoy walking outside I believe (for me, at least) a controlled environment is safest. Either a treadmill or stationary bike would be best for me. There's too many factors to consider being outside (uneven ground, temperature, bright sunlight, etc.). None of which used to be an issue for me...but now they're things I have to think about.

Resistance training using free weights is probably good as long as you don't overdo it, can hold onto the weight safely and don't get overheated.

I'm of the opinion that any exercise is good as long as you don't overdo it and can handle the pace. I try to incorporate housework and daily living activities into my "exercise" routine by walking and stretching as much as I possibly can throughout the day.