The Isabella Stewart Gardner Museum (at 280 "The Fenway" -- red star on map below) is a really quirky, unusual little museum -- a mansion built by a wealthy patroness of the arts to house her personal collection, including portraits of herself by John Singer Sargent,
Whistler
and Anders Zorn.
Also lots of European medieval art -- like the whole inside of a chapel plundered from some castle -- and a kind of lush garden setting, inside and out (The Fenway -- the street, that is, not the ball park! -- really is rain-forest-y marshland). It's been 25 years since I lived in Boston, so I can't venture a guess about "safe walking" -- back then it was potentially wild and woolly anywhere, anytime. But it's not far by public transportation or taxi -- not far from MFA either, I think. And if your wife's first name is Isabella, she'll get in for free! (told you it was quirky!).
Edit: maybe this link to map works better than thumbnail below: http://maps.yahoo.com/maps_result?ed=Z8poF...y=us&cat=&trf=0