As someone who recently made an effort, both mentally and physically, to take better care of my body, I completely agree with "something you enjoy".
I used to try to run, but I've never enjoyed running. As you might assume, it was easy not to do it often, if ever, and when I did do it, it was a struggle to find the motivation.
I do however enjoy cycling, so I have an $80 used mountain bike and it's a lot easier to take a daily cycle because it's something I enjoy.
Other simple suggestions could be gardening (it's actually a fair bit of exercise usually), or a brisk walk, or a slow walk, a hike or even a light swim. Anything to get your muscles moving and of course the other benefits, stimulating your mind.
Like
@sunkacola said, it's something I can do lightly on days I'm "not feeling it", and it's easy to do a few times a week when the weather is nice.