My favorite mansion was Marble House.

Newport is fabulous - you won't be disappointed. For a nice night out, head to the Clark Cooke House on lower Thames St. You'll probably need reservations so call in advance. Great place to get your fill of the quintessential Newport scene. You may also want to check out Scales and Shells a little further down the road for some excellent seafood in a more laid back environment. Even though it's warm out I'd recommend a trip to The Black Pearl for a cup of clam chowder just because it's delicious.

In terms of things to do, pack your sneakers and plan to spend an afternoon on the infamous Cliff Walk around the shoreline. All of the mansions are spectacular so you can't really go wrong. You might want to think about doing a Viking trolley tour as well, which is a nice informative tour of the scene all around Newport including a trip out to Ocean Drive to visit all the mansions. Lots of great tidbits about local history, too.

My absolutely favorite thing about Newport is just walking around downtown, breathing in the salty air and enjoying a bit of history. Definitely poke your head in at St. Mary's Catholic Church, which is the church where JFK and Jackie were married. Better yet, stay for mass if you're so inclined. When it's all said and done and you're back at home, go read Theophilus North by Thorton Wilder and fall in love with this great city all over again. Enjoy!

AKN, thanks for such a great, detailed reply! (That is why I love this website!) AKN, do you have any accommodation suggestions? (Thanks, also to Cramachin!)

I never stayed in any hotels in Newport since I was living in an apartment so probably not the expert here. I always thought the Francis Malbone House looked lovely. It's right downtown in close proximity to everything but be warned that it's expensive. To that point, most of the B&B's downtown will be expensive since you're now in peak tourist season. If you're renting a car, you might want to consider staying outside downtown Newport. Remember that you can drive the entire state in 45 minutes so nothing is really that "far" away! Another thought here (again, haven't done this but thought of it this morning) - depending on how long you're planning to visit, you might want to look into renting a cottage for a few days. Lots of people have summer rentals available and you might be able to get a better deal if you're planning on staying for more then a weekend. Good luck!