I find this absolutely fascinating. Is it that you are only thinking about the task you're doing at the time, or not thinking at all? And if it's the latter, can you describe it?
My brain is constantly up to stuff - minimum of 3 or 4 things at a time. For example on a short walk yesterday I had the following strands running around each other:
Immediate practical - need to be back by 2.30, so monitoring time, when to turn back from the walk, factoring in time to stop and buy bread.
Future concrete - there's a lot of seaweed washed up, so must remember to come back tomorrow with a bag to gather some for the compost heap.
Future speculative-practical - rather than buying bread I could start making sourdough again. Planning which culture to use, how to schedule it, recipe ideas, working culture feeding around holidays....
Observational - there's a big flock of finches. Are they linnets or have the winter twites come back? Oh look, there are some housemartins so maybe it isn't winter yet. They'll be migrating soon (bit of brain wanders off over their migration route, flying along with them, checking what countries they go over, trying to remember the flags of those countries. Another bit of brain then hops back to my childhood bedroom where I had a poster of world flags on the wall. The wall had woodchip wallpaper. Who invented that?)
Location memory - on a previous walk through the area a very chatty man told me the life history of a family who lived in one of the houses. So when I walk past the house I remember that conversation as a conversation. But also run through the story itself as a separate line of imagination - following the various people around the world.
Speculative-historical - how old are those houses, what would the town have been like when they were built, who would have lived in them? What would my life have been like as one of those people?
Nosy - can I see the decor of these houses through the windows? What's the rest of the house layout like, and how would I change it if I lived there?
Fantasy alternate reality - what if I bought a house in Greece? How would I make a living? What would I grow in the garden? If I had a pool with a biological filter system that meant I couldn't wear sunscreen to swim, would I only swim after dark or could I rig up a sunshade? But that would mean losing solar heat input into the pool. But you can get semi-transparent solar panels, so if you used those as a shade, and ran the power through a heat pump, how big would the canopy have to be for the electrical heat to get the same water temperature (run calculations assumimg 20% panel efficiency and 3:1 ratio from the heat pump).
Sensory-musical - weaving rhythms round the beat of my footsteps. And counting them.
Earworm - Magic Roundabout theme tune.
Verbal - constructing an email argument with our HR department.