Harvard/Harvard Square
Fenway. Even if the Sox aren't in town, it's cool to see.
Bunker Hill and the Constitution, for a sense of just how small the places were in which this nation won its freedom. It was hand to hand combat.
The North End for a cannoli (or a festival if you're in on a Saturday night in the summer). Mike's gets mobbed so hit up the Modern or Sporto up Hanover Street. Cannoli's just as good.
Take a walk over Beacon Hill.
On that note, just walk. Central/downtown Boston is like nowhere else in this country to be a pedestrian.
Do not drive anywhere. The T is a wonderful thing.
If you want good fish, find your way to place called The No Name, on the commercial fish pier on the South Boston Waterfront. It's a throwback. And (relatively) cheap.
If you're a runner, go for a run on the Esplanade, and over the Longfellow Bridge.
If you're a drinker (though you sound maybe too young), hit The Sevens on Charles Street.
Have fun. It's a great town.
- A proud native Bostonian