During the noon to 6 you can easily do the Udvar Hazy Air and Space museum thats is roughly on the SE part of IAD. It has a bunch of aircraft and one of the space shuttles. This was created in part because the air and space and a warehouses full of things they didn't have room to display. Also this has an IMAX theater.

For the 4-10...a couple of shopping options with restaurants and go see a movie. There is Reston and Dulles Town Center are the closest. A little farther away is Tyson's Corner ( 2 malls) and fairfax with a mall and a few shopping centers.