Hold up your left arm, like you're showing off your massive biceps. Use your right hand to point at your left elbow. Congratulations! You are now perfectly equipped to tell people where the wedding is.
Originally home to a tribe of Native Americans called the Monomoyicks, Chatham was explored in 1606 by Samuel de Champlain. The English arrived in force in the late 17th century, after William Nickerson moved from Yarmouth to an enormous homestead virtually the same size as the current town. Chatham was incorporated in 1712, but didn't really get going until after the Revolutionary War. It cleverly got out of the salt-production trade and into tourism in the late 19th century, when industrialization and its associated economic growth made seaside vacations the thing to do.
Being on the coast, it naturally has a Coast Guard station and lighthouse; if you get bored at the reception (and have packed decent shoes), it's a pleasant walk from the Clarks' to Chatham Light. If you're in town on Wednesday, there is a guided tour at 1 pm.
Also within easy walking distance is Atwood House, home of the Chatham Historical Society; in fact, it's right around the corner. The house, as well as the society's other buildings, is open Tuesday through Saturday in the afternoon, but opens three hours early when it's raining, which Sarah thinks is terribly precious.
Chatham's Main Street is lined with interesting shops. Sarah and Greg are fans of Yellow Umbrella Books at 501 Main Street, and friends of the family own Monomoy Coffee at 447 Main Street. Greg's mother and aunts often have to be physically restrained to keep them away from Chatham Presence (632 Main), Yankee Ingenuity (525 Main), and Tale of the Cod (450 Main).
Non-shoppers can visit Cape Cod National Seashore, play miniature golf until sunburned beyond recognition, bike along a rail-trail (Massachusetts requires helmets; you've been warned), or just plain go fishing. Or, if miniature golf and regular-sized fish are too small-scale for you, there's regular-sized golf and giant-sized fish-shaped mammals!
The following websites, listed from most-official to least-official, can give you a good start in planning any free time you have in the area.