That reminds me of another tip - there are many kilometers of underground arcades and passageways beneath that part of Tokyo (including underneath the Four Seasons Marunouchi), and I'm pretty sure you can walk from the northwest end of Otemachi station all the way to Higashi-Ginza station without going outdoors, assuming you can figure out the route. It's not all super-air-conditioned (though some parts are), but it's generally cooler than being out in the sun.