I can remember the first time I saw the Smithsonian Castle and carousel. It seemed like a fairy tale right there in the middle of DC. I can spend hours wandering around the gardens behind the castle. Most people skip by them, but they are a beautiful, serene gem right in the heart of the city. The castle itself features a scale model of the federal city which is not to be missed.
The real draw is the Smithsonian Carousel. Bring a picnic and let the kids run around and then ride the beautiful horses.
The Carousel is open everyday except Christmas (and blizzard days!) from 10 am to 5:30 pm.