Lowell National Historical Park encompasses the historic mill district at the heart of the city — a network of five canals, dozens of 19th-century mill buildings, worker housing, and commercial blocks that together constitute the most intact example of early American industrial urbanism in the country. The park is not a single building or site but an entire district you walk through.
The visitor centre on Market Street is the best starting point. Rangers run excellent guided tours of the canals and mills, and the orientation exhibit gives helpful context before you explore independently. The self-guided walking routes are well marked and cover the canal system, the working-class Acre district, and the commercial centre. The combination of the Boott Mills museum, the canal tour, and a walk through the Acre takes a full day and rewards it.