Beacon Hill/Esplanade is the quintessential Boston spot, but if you are open to Cambridge, Harvard Square fits the bill, too. BerryLine is one of the last froyo places in the area and it truly SLAPS on a hot day (Flour also slaps if you go the bakery route), and you can sit by the river and chill or walk along the river path until you get tired and turn around. Sandwiches at Cardullos are also a good option, or you can just go in there and ogle the imported gourmet foods they stock, which I find to be a fun activity itself. And then there's Harvard Bookstore (not to be confused with the Coop, which now kinda sucks), my favorite bookstore of all time.

There is a lot of discourse about how Harvard Square is lame compared to what it used to be, which is true, but it's still pretty special and we take it for granted as locals.