Some of you know that I have lived in several places of the U.S. yet moved back to the Washington, D.C. area. Having lots of FREE museums and parks to attend was one of the deciding factors. The United States National Arboretum is one of those places. Open every day from 8 am - 5 pm except for Christmas day (Dec 25 for you Russian Orthodox folks), it is completely free: free admission, free parking, free restrooms, a good portion of the place is handicapped accessible, and you can bring your dog!
Established in 1927 by an Act of Congress. The Arboretum is administered by the U.S. Department of Agriculture’s Agricultural Research Service with 446 acres and 9.5 miles of winding roadways.
You can watch this video about the Arboretum (in English with subtitles and they speak REALLY slowly!!)
You may remember this photo from another thread... it was taken here
Here is another view of the columns...
and one last view...
The have a very large Koi Fish Pond and in the Spring and Summer you can feed them
They also have several areas, like the Asian Garden Area complete with a small Pagoda
They also have the National Bonsai & Penjing Museum
There are many, many, many walking/hiking trails with wooden bridges and benches along the way to rest or just chill out.
here is a link to someone who has posted several photos from the azalea trails
This one is from Fern Valley
Map
It is off the main area of tourist attractions, so if you do ever come to this area, you would have to make a special trip to visit.