The usual answer is: start a journal. It’s a good way to simply let you mind wander while you get back into the creative stream of writing.
I’ve never had a journal. Tried it several times and it never worked for me. In fact, I hated it.
For me, simply taking a walk does the same thing because my mind wanders all over the place as I move along a trail/road/path. Usually, by the time I’m back home, I have more ideas than I need and can hardly wait to get them down on paper/computer.
So what do you already do that piques your imagination? Gardening? Fishing? Cleaning closets? (Yes, I know someone who finds inspiration while cleaning closets.) Whatever?
The only real requirement for whatever you do is that you do it in solitude so it’ll be your thoughts that come to the fore and yours alone.