@burialchoice Looking at my family, I'd say stretching and just general movement.
My Dad is in his 50's and has always been active and had a workout routine for as long as he can remember. He's in great shape, but he says immobility from being inflexible and is already kicking in. He never prioritized consciously and intentionally stretching until a few years ago. He has to warm up for a long time before working out and has to really cool down and stretch thoroughly afterwards or he'll be stiff and sore within the day, even for a relatively easy workout. On days he doesn't work out, he notices how stiff he gets, especially in his legs and back. Meanwhile he has a friend who is an avid yogi who is still very flexible and mobile.
As far as movement, my grandparents are both 91. My grandpa was going to the gym to lift weights and swim laps up until he was 89 (and he only stopped because of Covid, he will likely get back to it soon). He still walks daily, maintains his garden, stretches every morning, and does simple dumbbell exercises at home with no signs of slowing down. Meanwhile, my grandma was always fairly sedentary. Now she can only walk/stand very short distances even with a cane or walker and has trouble getting even basic things done. It's sad to see her like this and the difference between them is stark.
I have a pretty intense exercise routine right now, but when I'm busy I feel good as long as I get a walk in and get a stretch in. Even if it's just a 30 minute walk on my lunch break and a quick 5 minute stretch before bed!