I joined a rock 'n roll band playing saxophone, also do a lot of work with a big band and help out with some very enthusiastic teenage musicians. I have also now joined an orchestra to get my clarinet playing back to some level of competence. I am now doing publicity for a music festival and various web pages etc. Oh - and I am also doing a degree at university (but I started before I retired). In my spare time I have started remodeling the garden and also belong to several specialist gardening clubs.
Retire? How did I ever find the time to work?
The secret is to start getting active in some interests before you retire, not after.