Great choice. Just walking down Thames st is entertaining. There are lots of shops, pubs and restaurants. You can get a combination ticket and see several of the mansions, all are gorgeous. Drive out along the shore, take a walk on the Cliff Walk (in front of the mansions) (for the uninitiated, the "front" of waterfront property isthe ocean.) The Castle Hill Inn has a great Sunday brunch and fabulous views.
Stop at the Tourist information center and you can get helpful info, maps, and mansion tickets.

Thank you so much for the advice! Anyone have any tips on restaurants??

Your B&B hosts should be able to help with recommendations, too. I spent a relaxing weekend in Newport in early March. Despite a Sat. night snowstorm, just walking by the historic colonial homes (up the hills) off Thames St. was interesting. I also toured 2 Mansions (Marble House and Chateau sur Mer - both GREAT) and wished I'd had time for more. The Cliff Walk is most interesting closer to the "Forty Steps" overlook, where the mansions are; there are only ocean-views (nothing much on land) between the Beach area "official" starting point and the Forty Steps overlook. Most restaurants feature seafood (which I enjoyed), but all seemed to also have vegetarian selections. The Black Pearl pub-side was romantic and reasonably priced (waterview side is considerably more expensive). The upstairs area at the Red Parrot was a nice place for dessert & coffee.
Also worth doing - Ocean Drive to the Brenton Point State Park.

More restaurant ideas?

I was in Newport last October and will be going back again...it's a great place! I ate at the Mooring and enjoyed it, would have no hesitation to return. It's on the waterfront and the food was good. If you only have time to see a few of the mansions, I would rec. Marble House. If you're interested in Kennedy nostalgia, Hammersmith Farm is a neat place to see. Jackie Kennedy grew up there and it was the site of JFK's and Jackie's wedding reception. Also, when I was on the tour last year, the guide mentioned that this year would probably be the last year that Hammersmith Farm would be open to the public because the new owners would be turning it into their private residence. Hope this helps!!

Melissa:
There's a lot to do in Newport. The mansions are a really fun thing to experience. The most amazing of them all, however, is the Breakers. It is fantanstically, ridiculously, beautiful and the most famous of all the mansions. Your innkeepers will definitely have different menus at your inn (there are so many restaurants there)(all the inns collect menus for their clients), so you shouldn't worry about finding a good place to eat. The Whitehorse Tavern is a great and romantic place for dinner, although I'm not sure about how fab their vegetarian selection will be. If you go to the library or the bookstore, they have a bunch of books about Newport and what to do there. I have a good one that has a small section on Newport that's called: Weekends in New England. It's a great book for weekend trips and gives the low-down on what to do, when to go, etc.

By the way, if I remember correctly, the Francis Malbone House is in a great location (Thames St is the "main street" of Newport) and looks very nice from the outside.

The Cliff Walk is beautiful, definitely do that. The rocks/ocean are stunning.

Enjoy your Newport weekend!