I suppose every big city has its hidden gems. For all the touristy things we've done here in Washington DC, I had never thought to visit the Library of Congress. It wasn't until a friend of mine started volunteering there that I'd even heard of it. Two visits later, and now I recommend it to everyone, because it's absolutely breathtaking! And that's just the visuals. As an avid reader and geek, the trivia and information is also amazing.
I cannot even describe the amazing architecture and beauty inside; I wanted to fill this post with 20 photos but decided to just give you a hint to entice you to visit on your own. The stories we heard while touring the Library told of the history of Washington DC, of art and science, and of how the Library is adjusting to the digital age. It was truly fascinating and was a wonderful visit for all generations (I took my kids and my mom).
Free guided tours are offered Monday through Saturday and last one hour. If you're an individual, family or group with less than 10, you can just walk in and join a tour. For a larger group, you'll need to schedule a group tour. I recommend you get several families together and request a docent who's good with children. The LOC suggests the tour for fourth grade and up, and I agree. If you plan to visit with younger kids, you may want to just self-tour. Check out the Activities for Kids & Families page for printable activities for younger kids to do while you visit.
The Thomas Jefferson collection was especially meaningful to us because we had visited Monticello and learned the story of his personal library, so my kids made the connection when we saw the efforts to replicate his books at the Library of Congress.
Be sure and visit the Young Readers Center, where my kids could have spent the whole day curled up reading. It's filled with galley copies of new books (before they're published in stores!) and something they were awed by–Braille copies of all of the Harry Potter books.