Beacon Hill is small enough to walk end to end in fifteen minutes, and the whole place is laid out wrong for the sun in the best possible way. The streets run east–west across the slope. The buildings are four to five stories of dark brick. The sidewalks are brick too, which holds a few degrees cooler than the asphalt one block down on Cambridge Street. In July at noon, walking up Mount Vernon from Charles you can feel the air drop as you pass Brimmer.
The picks below are graded for midday. Most use the south sides of the cross streets for cover from the building line — the trick on the Hill is to commit to one street and stay on it; the cross-block jogs through alleys (Acorn, Cedar Lane Way) are pretty but they put you back in the sun for a beat. Stay Cool will route you around them if you ask.
One caveat: the bricks make some of these walks rough. If you are pushing a stroller, the route through Louisburg Square is going to feel longer than the eight minutes the app shows.