Mount Vernon was supposedly the thing not nearly enough people see when they visit DC, so as a history teacher I had to make sure we made the short trip into Virginia on our recent family vacation to the nation's capital.
I was so impressed.
First of all, as you may have heard, the Mount Vernon estate is beautiful. Even if every single building on the grounds were closed and all there was to do was stroll around, it would be worth the trip. Make sure you build in some extra time to see the gardens, pastures, hills, wharf/waterfront, woods, everything.
What's more, George Washington's estate has been remarkably preserved. Remarkably. There are numerous free tours with different themes offered throughout the day, there are guides and other helpful people around the estate in period costume, and all of them know an immense amount about the Washingtons, their lives, their era, the grounds, the buildings, everything.
For me, the history teacher, the really breathtaking thing was how much of the estate is authentic. These were George's actual wood floors, the harpsichord his step granddaughter played, the key to the Bastille given to him by the Marquis de Lafayette, the chair in which he wrote letters, the bed in which he passed. Nothing reminds you how young the United States really is than looking at the chair in which your first President sat while penning hundreds of letters. It is still covered with its original leather. I was blown away.
Mount Vernon also offeres a fantastic education center and museum, including several likenesses of George Washington (at different life stages) that took an immense amount of work and research to produce. You will learn (and be moved) without being buried in minutia. There is also a clean food court with good variety (very handy since you may be coming from DC) and a gift shop that offers some very nice products. Sadly we missed out on the George Washington rye by just a few days - it sells out fast.
If you're vacationing in DC, I highly encourage the (short) trip to Mount Vernon. Make sure you budget some time for it; you will be sorry if you have to leave too soon.
(One additional note - the manor house itself is in original condition, which means no wheelchair access to the upper floor(s).)