root beer float
STRONGLY seconded - Cabot's is a great old-fashioned ice cream parlor and they do a fine job with the old-school fountain treats.
I can't remember whether I've had a root beer float there, because I usually go with an ice cream soda (there's hardly anyplace else left in Boston that has any idea how to make one!) but I will enthusiastically vouch for the quality of their offerings.
(Except the raspberry lime rickey, for some reason - DO NOT get a raspberry lime rickey at Cabot's unless you have a weird nostalgia for Za-Rex syrups.)