Retirement is like marriage, you have to work on it. Just leaving work, saying what next and waiting for life to come to you is a recipe for boredom and misery.
The classic occupations are to get involved in voluntary work. Now one person's voluntary work is another persons purgatory, but I just googled 'My town volunteer centre' and the number and variety of volunteer opportunities that came up varied from school governorships, helping with a food bank, working in a charity shop, mentoring families and young people and many more. There is also the Mens shed movement.menssheds.org.uk/ This is exactly what it says a large communal shed where men get together for a bit of company and to do some of the things men do in sheds.
I do not know whether you live in a village, town or city, but your local library often has directories and lists of clubs and organisations in your area and you may find one that interests you. Ask your local Age UK, they usually arrange regular events and activities, it is not just day centres. They will have drop in coffee centres, they organise exercise classes and IT classes. Just go in and ask.
At the end of the day the ball is in your court. You need to look around at what is available locally, do not reject everything, choose one, then take hold of yourself by the scruff of the neck and take yourself there.