A young colleague gave me a retirement pack constructed by her mother. A book, a large box of chocs and a bath bomb. Mum's advice was "breathe". Take a minimum of a month, and at this time of year, plan a Sept holiday, splashing out.
I did not retire all at once - I worked at the college as an exams invigilator, on call casual, I joined the elections team as a poll clerk, I looked at town clerk role, but decided too much work.
I am active at my local school as a reading helper which fixes my calendar to one day a week - I find it important to have something regular in my schedule.
If it floats your boat, your local library will be able to signpost volunteering activities, even with them. I recommend GirlGuiding who are always short of leaders and you get to be part of a mixed age team
If not, there are literary clubs for budding writers, plenty of craft clubs, Trefoil Guild (you get to meet your neighbouring community which, if working you may not be quite part of), for older retirees there is WI and U3A. If you still feel in"work" mode, you can always become part of the organising committees.
Or you could take to politics with your local town or district council.