Shopping on Newbury Street, Prudential Center, and Copley Square (between those three destinations, you'll have all price ranges).
Isabella Stewart Gardner House/Museum--right across from the MFA and much more intimate. Very romantic place!
Italian food on the North End. The original (ie, not the mall chain) Pizzeria Regina is there, and amazing. My best friend works at a restaurant there, he says it's the best seafood place on the North End (seafood imported from Sicily). If I talk to him soon, I'll PM you with the name.
If you just want to chill for a few between hitting stores on Newbury, the Trident Bookstore has a great cafe with affordable sandwiches.
I'm just generally a big fan of the Back Bay and Fenway.