What kind of places you like… historical or like some other places,… boston is like mix of all for all tastes..
The freedom trail is good start, then a tour of charles river, which can also be done through Boston Duck Tours, which like amphibious vehicles, which moves around whole of Boston, and then takes a dip in the Charles River as well.
Fanueil Hall and Quincy Market are good… Alongwith many of the Museum and Aquariums.
The whole longwood area is famous for Harvard MIT and a lot of other colleges/universities.
The famous reflecting pool is good as well.. http://upload.wikimedia.org/wikipedia/commons/3/3e/USA_09562_Boston_Luca_Galuzzi_2007.jpg
Btw all these places are very near to each other and all are acessible by Subways , known as the T.
See this site as well. Boston
