I've visited the 9/11 Memorial a couple of times so skipped it on this trip; One World Trade Center (ie Freedom Tower) is still jaw-dropping as the tallest building in the Western Hemisphere (the same architect designed Willis Tower and Burj Khalifa)
The Cooper-Frost-Austin House, built in 1681, is the oldest house in Cambridge; the city of 116,000, home to Harvard and MIT, has 57% of its residents possessing at least a bachelor's degree
The First Church of Christ, Scientist is considered the Mother Church for the Christian Science church and was completed in 1894 with major additions in 1906; it boasts one of the world's largest pipe organs and has a sanctuary that seats 3000