My favorite: The Duck Tour - Tour Boston on a "DUCK", an authentic, renovated World War II amphibious landing vehicle. You'll cruise by all the places that make Boston the birthplace of freedom and a city of firsts, from the golden-domed State House to Bunker Hill and the Fleet Center, Boston Common and Copley Square to the Big Dig, Government Center to fashionable Newbury Street, Quincy Market to the Prudential Tower, and more. A great 90 minute tour of Boston and its history. Warning: These are very popular and you usually need to buy tickets in advance!!!
The Boston Visitors' Bureau - Listings of things to do in Boston includings tours, museums, theater etc.
Boston Visitors Guide - as above!
Prudential Center Skywalk - 50 floors up, this offers great views over the whole of Boston. If you're not too bothered about walking the whole way around, but still want good views, I would recommend going to the 'Top of The Hub' bar and restaurant on the next floor up - if you're going to pay $6 to walk around you may as well may a little more and have a drink! Besides, they do one of the best dirty martinis in Boston (ask for the Level vodka!). Food is expensive - never tried it but it is supposed to be good. And did I mention that back on the lower levels of the Prudential center is the biggest shopping mall in town?
New England Aquarium - Supposedly one of the best aquariums in the world. Everyone that has been tells me its great. Fantastic for kids!
The Freedom Trail - a 2.5 mile red brick or red painted line that travels through Beacon Hill, downtown Boston, the North End and Charlestown and takes you through lots of historic sites. Plus it goes through the North End - the Italian district which is a great place to stop for lunch! Also goes close to Faneuil Hall and Quincy Market - also Boston landmarks.
I could go on, bit you can probably find all the information you want on one of the visitors guide links. If there is anything specific that you want to know, just ask us!