We recently visited Mt. Vernon. It costs about $15 to get in, but it really is worth it. We went once but only had about 30 minutes before it was going to close so we didn't go in. This time we did the full tour. We saw the inside of the main house, GW's tomb and all the fixins.
They recently finished a fabulous museum that includes these spot on replica life size models of George Washington at various stages of his life. They were very cool. I would love to get permission to go in with all my gear and do some more portrait-y portraits of these things.
They also have a set of George Washington's dentures. CRAZY! They don't allow pictures to be taken of them, but they are lead with animal and human teeth. Of all the crazy interesting stuff there is to talk about at Mt. Vernon, the dentures are what I thought about most on the way home.
If you ever visit Washington, DC or Arlington, Virginia, I highly suggest spending a couple hours out at Mount Vernon.