Canyon Ranch near Lenox might fit the bill. Prices include meals.

http://www.canyonranch.com/lenox/

I'll second the recommendation for Canyon Ranch. Great experience.....but I warn you that it's expensive! My wife and I have been there twice and had a great time both times. We were even in the same exercise class with Joanne Woodward!

http://www.tripadvisor.com/Hotel_Review-g33878-d85822-Reviews-Saybrook_Point_Inn_Spa-Old_Saybrook_Connecticut.html

mclaurie, the Old Saybrook faciiity may indeed "fill the bill," but it would require a signigicant driving detour from the Boston/Rochester route.

Agree about Canyon Ranch.

I recommend the Gideon Putnam in Saratoga Springs, NY, just a little detour north of I-90 on I-87. The hotel is historic and gorgeous, on the edge of town inside a park. The grounds include the equally historic Roosevelt Baths spa, which I understand have been newly renovated. (The website even has a link specifically called "Girls Escape Package"!) If you want to venture into town, there's everything from great restaurants to shops and nightclubs.
http://www.gideonputnam.com/

I've done a girls weekend at the Elizabeth Arden Red Door Spa, inside the Mystic Marriott just outside Mystic, CT. It was a nice, mellow weekend - and a great massage - and Mystic is close enough for an afternoon of browsing the shops and a light dinner after. But like mclaurie's suggestion of Old Saybrook, it's farther off your route.

Thanks everyone. Great suggestions!