Boston is one big tourist trap this time of year. Tons of families exploring Downtown Crossing and that area, lots of historic museums and several Revolutionary-period buildings if you're into that sort of thing. I've been visiting them on my lunch breaks actually, haha. The Duck Tours are pretty entertaining especially if you've never been on one. There's a ton to do in Boston, though it can be a bit daunting if you're a townie at heart like me. If you like old places, Old Sturbridge Village (or just Sturbridge/western MA in general) is pretty scenic and very interesting too. I haven't been to OSV in many years now, we used to go as a family several times a year when I was a kid!
Play to your interests - I think it's fairly evident mine is historic architecture. You probably get enough of that living in Newport though.
Some questions might aid you: What is it you'd like to take her out for - shopping, rock climbing, hiking, pictures, etc? What are her own interests (i.e. would she find it as fun as you would)? Are you planning to surprise her or plan this out together? Depending on personality a random trip anywhere can be either outrageously fun or regretful.
