What activities facilitate deep thought for you?

A lot of my software engineering design is done in my head, on long walks. When I sit down to write code and get stuck on deep design issues, that's what I do.

Sleeping on problems and thinking about them in the shower is also quite productive, but of course one cannot think for long periods while engaged in these.

What are some activities, that one can do for hours, which facilitate deep thought while engaged in them, while also being useful in themselves?