The Charles River divides Boston from Cambridge and each autumn the Head of the Charles Regatta takes place. The two-day sporting event includes more than 8,000 rowers competing in over 50 races and draws more than 300,000 spectators (many of them college students proudly -and somewhat drunkenly- cheering on their school).
The seven bridges that criss-cross the Charles River along the race course provide excellent vantage points, but I'm more partial to viewing from the Cambridge side nearer Harvard University and Harvard Square. If you are new to Boston or rediscovering the city after many years of calling this place home - I would strongly recommend that you come and watch the Regatta. Harvard Square pubs and streets are overflowing with people and the city is abuzz with rowers who have come from all over to compete in this prestigious and largest regatta in the world.
The Boston Globe has an excellent article, full of photos called, Head of the Charles 101.