I'm sure that I would have the same issue if I were to retire today. That's one reason that I am planning to keep working for a few more years and then perhaps ease into RE with a part time status for a little while. That should give me more time to thing about how to approach RE life.
Now, as to strategies to combat un-enjoyed idleness, I think perhaps the best course is to try lots of different things. Try things even if you can't think of anything that seems thrilling to you right off. Just try lots of things, like dating lots of people, knowing that most of them won't work out.
Don't limit those things to unpaid 'hobbies' either. You are allowed to work part time someplace just for the experience even if you don't need the money.
Maybe just start with a little brain-storming and list out a bunch of things - even things that you wouldn't necessarily want to do, just things that you could do. And then pick a couple of them and try. Volunteer at a community theater to help build sets or something. Or volunteer at the local library a few hours a week. Try to write short stories. Take a scuba diving class, if the budget allows. Learn to cook by picking a few recipes and trying them. Join a wine club.
This passage is attributed to the German writer Goethe. It's debatable whether this is an accurate translation, but I like it:
"Lose this day loitering—’twill be the same story
To-morrow–and the next more dilatory;
Then indecision brings its own delays,
And days are lost lamenting o’er lost days.
Are you in earnest? seize this very minute–
What you can do, or dream you can, begin it,
Boldness has genius, power, and magic in it,
Only engage, and then the mind grows heated—
Begin it, and the work will be completed!"
Point being: just start doing something. The actual doing leads to more motivation to keep doing whereas waiting for motivation before you even start can often be fruitless.