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 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