^East Potomac Park- There are usually plenty of free parking unless there's a major event on the Mall. It might be a long walk for some though (I've seen families do it still).
I'd avoid the munchies in the museum
Most of the souvenirs can be found in touristy gift shops around the city for cheaper (a bit) especially if it's a bit outside of touristy areas like one of the malls (not counting Pentagon Fashion Centre
). Though I always wanted to buy stuff at the museum of natural history, especially some of the books.
Gee. I miss DC