I wear something like the wasteland wanderer outfit from fallout 3.
Lightweight linen cargo pants, light weight t shirt or short sleeved rash guard (like a tshirt made of something like women's bathing suit material, some claim to be spf 100 (I don't surf)), and a light weight hoodie. I never wear sandals or have bare legs in public even though that's the norm where I live.
In the winter I wear pretty much the same outfit made out of heavier materials.
I'm a tiny adult, I get my clothes in the kids sections but they're cheaper so whatever.