Take Branch Avenue (Maryland Route 5) out of the District, being especially careful when you get to Waldorf: In Waldorf they try to take Route 5 away from you, leaving you on U.S. 301. Take a left here, staying on 5, toward Leonardtown. Shortly after Hughesville, noted for its tobacco auctions, you'll see Charlotte Hall on the right. This is the time to get your brochures - including one on the archaeological park and the Potomac museum.
If you want to go to the St. Clement's Island / Potomac Museum first, take a right on Route 242 and follow it down to its end, then follow the signs. The museum's open daily, 9 to 4, and costs 50 cents for adults (children under 12 free).
If you want to go to the dig first, stay on Route 5 through Leonardtown, then take a right on 244. About two miles along, right after the gravel pit, you'll see signs directing you to the site, which is only about a mile off 244. It's also open 9 to 4 daily, through September. Admission is $1 for adults, 50 cents for children. For information on both places, including arranging the boat trip, call 301/769-2222.