Concord's attractions divide neatly into two categories: the Revolutionary War sites clustered around the North Bridge, and the literary heritage preserved in the houses of Emerson, Thoreau, Hawthorne, and Alcott. Most visitors try to do both in a day, and that is quite possible — the town is small enough that everything is walkable or a short drive.
The Minute Man National Historical Park covers the key Revolutionary sites; the Concord Museum provides the background context. Walden Pond is 2 miles south of the town centre, a 20-minute walk or a short drive.