I had a rough idea of what I would do after ER. But for a good year before I pulled the plug I worked on a list of activities to consider, pursue, actually do, etc. Consider making such a list. DON'T pre-edit, just write down anything, any activity, any subject matter, that is of some interest, or might be of some interest. Then, you can start exploring how each item on the list could translate into a retirement activity.
Not all will work out, but the exploration can be fun. For example, bird watching was on my list. I joined a group and did it a few times. It was nice, but ultimately not for me. Flip side was my plan to get back to playing competitive Bridge. That has been a resounding success for my retirement lifestyle.
Retirement activities need some planning to make sure you enjoy it.