I always love finding a calm place to sit down and relax for a bit, and when the weather isn't way too hot Idée Café Parc is a pretty decent place to go with its fairly large terrace overlooking the plaza right in the middle of Tokyo Midtown. They used to have this super tasty belgian waffle thing, but sadly, the last few times I've been there the waffle has been absent from the menu.
Still, a great place to unwind.
Tokyo Midtown, 3F, right on top of the main entrance. It's not immediately visible from the inside, but just walk through the Idée Shop store and you won't miss it. Store hours are 11:00 - 21:00.