I've found that swimming helps me to do that. When I walk, it's too easy to take my kindle and read but swimming, it's a relaxing and repetitive movement where I can only focus on thinking.

Same! Although nowadays our sea is hitting record temperatures (28C [~82F] in the water today), and the jellyfish seems to be enjoying it a lot, so swimming is now 50% "relaxing and letting my mind wander" and 50% "shit, that's too many fried egg jellyfishes, lets swim this direction oh no another one"