Walk the scenic trails of Niagara Falls State Park, admire the majestic waterfalls, and take the classic Maid of the Mist boat tour (seasonal, check schedule).
Visit must-see Smithsonian museums such as the National Air and Space Museum and National Museum of American History. Most museums open at 10 AM and have free entry.
Stop for sightseeing and a light hike in Shenandoah National Park. The park is open 24/7 but visitor center hours vary; outdoor hiking is always available.