What to see in Boston?

My sister-in-law is amazing at getting bargains and she called me last night to tell me about something called the Go Boston Card. We are in Boston for the weekend for a family wedding and my six-year-old son is with us. I want to do some sightseeing, and this card apparently gives you admission to all of the events without having to pre-purchase entrance tickets. It’s pricey, but for some of the bigger tours like the Duck Tours (your “bus” is an amphibious vehicle) and the Beantown Trolley, by the time we pay our fees for those, we’ve already paid for the card. I figured, why not?

So, I’m debating on what to see. Probably the USS Constitution and Beacon Hill. Have you ever been to Boston? What would you recommend?