Follow the Freedom Trail and you will end up in the North End. Lots of restaurants on Hanover St and Salem St. The Daily Catch is great - small but really good seafood and cash only.

Closer to your hotel is the Atlantic Fish Co on Boyleston and there are a number of Legal Seafoods in the city (not my favorite but very popular). Turneer Fisheries is pretty good too. Personally I find all of the seafood restaurants in the vicinity of the Back Bay to be overpriced. I know there are others in Cambridge, Kenmore Sq and the South End that others will recommend- i just haven't had the opporunity to venture there.

Make sure you walk through the Public Gardens - spring is when it is at its best. You will undoubtedly see lots of brides having their photos taken. A walk along the Esplanade is great too - if the weather is right you will see lots of sailboats and sculls.

If you are an art lover, check out the MFA - I love the new American Wing. Haven't checked out the new addition to the Isabella Stewart Gardner Museum yet but have heard good things. If your name happens to be Isabella you get in for free.

Have a great trip.

The weather could be anything. I think shorts are a little too casual for the city but I'm an old fogey.
On Friday and Saturday, the push cart vendors are in Haymarket Square. This is adjacent to Quincy Market. Across the street the Italian North End starts. Walk thru to experience an old tradition. Saturday will be mobbed. Friday is better.
If you like Chinese food, try a dim sum meal in China Town or just visit a bakery for some interesting pastries. I have been on both the North End and China Town tours with Michele Topor's culinary tours. Both are great, esp because you learn about the history of that particular neighborhood. Trouble is, you only have 2 days and one tour takes up a good chunk of the day. Lots of food samples.
The Kennedy Library is great, too, and accessible by public transportation.
Logan expects you to be there 2 hours ahead. It's a big airport and the lines for security can be long. Don't cut the time short. If you don't have much to carry, you can get there by subway.

You may want to split up the Freedom Trail into 2 days; esp if you want to do the entire thing.

You can do Freedom Trail on one morning, then spend the afternoon in Harvard Sq in Cambridge.

Then Freedom Trail the next morning, and Museum of Fine Arts in the afternoon.

Turner Fisheries has pretty good seafood; it's inside WEstin right next door to your Marriott. If you don't mind walking about 10 minutes to Kenmore Square, I highly recommend Island Creek Oyster Bar for seafood.

Shorts is okay for walking around BUT it may NOT be warm enough in mid-May to wear them! I'd go with jeans and khakis. Even if it's warm during the day, it cools off quickly when the sun goes down.

On Mass Ave in Csmbridge (Bostonians abbreviate everything) is a small seafood place called Dolphin Seafood. It is between Putnam Ave and Harvard Square in the mezzanine floor of an apartment building on your right. It is classic NE seafood, very well prepared and inexpensive. Very reliable. When my wife was a student and we were too poor to eat out more than once a year (yes!), this is where we ate.

The weather in Boston can change on a dime and this winter has been even weirder than usual. Just be sure to check the weather report before you arrive. You see all manner of dress so whatever you decide should be fine. I wouldn't wear shorts to dinner though.

I'm with yk on the recomendation for Island Creek Oyster Bar. It's really good but it's not inexpensive by any stretch. Legal Seafood is usually pretty good.

The airport will be almost empty at 9:00 for a domestic flight on a Saturday night. My guess is an hour should give you enough time to get through security and make your flight.

Mid-May is typically rainy in Boston and it can still be cool (as others have said), so definitely check the weather forecast, even up until the day before your departure, and pack accordingly.

What the flora will be like in May is anyone's guess. Things are blooming several weeks ahead of schedule (even as much as 2 months ahead, it seems to me) thanks to the warm winter. A hard freeze earlier this week damaged the flowers on some flowering trees, but other plants do not look like they suffered much.

There are companies that offer guided tours (by costumed guides) of the Freedom Trail. The National Park Service also offers a free guided tour of part of the Freedom Trail. You sign up for it at the NPS headquarters near Faneuil Hall. A number of sites along the Freedom Trail charge admission, while others are free. For complete info, check out

http://www.nps.gov/bost/planyourvisit/index.htm

http://www.thefreedomtrail.org/visitor/visitor.html

Boston weather in late Spring can be unpredictable, to say the least. Come prepared for anything! Best advice is to dress in layers, especially if you plan to be out for the day. There can be quite a range from morning temps to those midday.

Last week we hit 83 degrees with beaming sun, and on May 22nd last year I ran a race in 34 degree, overcast conditions. It is THAT wide open. It should be nice, but be ready for cold or rainy, too.

The Freedom Trail can be done in a day, but it is a LOT of walking. Breaking it up over your two days may be worth it. With just two days, I could go either way on this. If you are fairly active with good energy, you can handle it in a day fine. Some great, great spots along the trail. Some are well known, some less-famous, like the Granary Burial Grounds (my favorite).

I also strongly recommend the Isabella Stewart Gardner Museum for art, a beautiful museum and a fascinating crime story surrounding it too... the empty frames still testify to the robbery years back! MFA is great, too. If you are a sports fan, you can get a tour of Fenway park. If you enjoy aquariums, the New England Aquarium can be fun to explore and the Harvard Museum of Natural History is kind of a cool spot, too.

Lots of good seafood recommendations here so far. I used to think Legal Seafoods was vastly overrated but have changed my tune. With the rise of strong competition, they stepped up their game in recent years and are a very good bet for great local seafood now... just a bit pricey. Still, if you want the traditional "Boston" experience that is probably the place to go.

Have fun! You'll love it!

Thank you all for your input, can't wait to go.

So glad to hear The Dolphin is going strong. It kept me and my future wife well fed during the blizzard of '78.

Will heavily second Dolphin Seafood as very good and reasonably priced.

Another good and reasonably priced seafood option is Yankee Lobster in the Seaport area. Kind of like a casual lobster-pound place, though no view. But prices are reasonable and items are fresh.

More expensive but excellent spots have been mentioned above, to which I'll add Neptune Oyster and B&G Oysters to the list.

Good sightseeing recommendations so far, especially if you have limited time available.

You are going to be there the weekend of graduation for Boston University and many other schools. Might consider making some dining reservations now, as there will be LOTS of families eating out that weekend.