As with others, got rid of mortgage and boosted pension, saw my parents, etc.
I made sure I visited my friends, all over the UK and some of the abroad ones, or made time for them to come to me. This hadn't stopped in my 60s, it is just on pause from Covid. But we're all getting older, and it is less easy to travel, sleep on sofas or strange beds, and some of my older friends are no longer here.
On that note, I would say get acquainted with Zoom, Skype, or similar, but I expect most people have now, as a result of Covid.
Got into some interesting things with like minded people... in my case, it was environmental stuff, witchcraft and tarot, but the basic thing is to develop hobbies, beliefs or pastimes that will give you outside interests, continuing personal development and help you keep going out and meeting people.
Visit some of the places you love, or always wanted to see, especially the less easy to access ones.
It has become apparent that I will never see the top of Glastonbury Tor or Croagh Patrick again, as I can't manage the climb, and I had my last trip up to Lud's Cathedral the summer before last.
I wish I'd done more to get the house ready for older age (although luckily a small legacy will take care of this) and that I’d got optimally fit, given my heart condition.
If you're thinking about retiring to some picturesque seaside or countryside place, do it sooner rather than later.
You'll be moving away from all your friends and contacts, that really good plumber, who always comes out, that trustworthy garage, the nice handyman, etc, and you'll have to find them all over again.
I didn't have grandchildren when I was in my 50s, but Covid permitting, will spend as much time with them as poss.