Lower Manhattan has a reputation for being expensive, and some of it is. Observation decks run north of $40. Harbor cruises add up fast for a family. But the neighborhood we live in is also one of the few parts of the city where the best stuff is genuinely free, and it's dense enough that you can do four or five of these in an afternoon on foot.
We've listed everything below with a rough walk time from John Street, since that's where we are and it's a useful center point. A few notes before you go. "Free" here means free to enter or ride, no ticket, no reservation. Where a place has a paid part and a free part (the 9/11 Memorial and its museum, Fraunces Tavern and its museum), we say so, because that trips people up constantly. And hours change, especially at the National Park Service sites, so check before you make a special trip.
If you're staying with us, the practical version is this: pick two or three from the harbor end and two from the Wall Street end, do them in one loop, and come back up John Street for lunch. You'll have seen more of old New York than most people manage in a week.