Many people are suggesting ‘walk’ itself as the answer, but this isn’t true for me at least. A physical walk is mentally like sitting around on a couch. My mind doesn’t really move unless I give it something specific to do. Cooking is a balanced mental+physical activity for me.
-Among people who know Chinese, it might be calligraphy (with brush and ink).
-This exercise is practice for maintaining the "running count" for blackjack card-counters / "Advantage Players":