I have definitely added the Massachusetts Historical Society Library to the places I would love to visit. I was reading their newsletter recently and they truly have some amazing items in their collection (and I'm sure it must have that wonderful old library smell).
Fortunately, for those of us who can't make it there in person, they have a pretty amazing collection on-line as well. For example, they have just put up the Online Adams Catalog - a comprehensive record of the papers of John Adams, Abigail Adams, John Quincy Adams, and other family members. It includes every known Adams family document from 1639 to 1889. How cool is that! Imagine the possibilities.