The Dulles 2 Dulles bus is(Dulles Airport to Dulles Town Center Mall and Air and Space Museum) now one dollar.
Like IAD Flyer said, go to Reston Town Center but if you don't want to spend 20 dollars+, you can take the 981 Fairfax bus that takes you to Reston Town Center. It is under 20 dollars each way per person. Plenty of nice resturants and shopping area.
For the noon-6 pm layover take Dulles to Dulles bus to the Air and Space Museum. Despite working at IAD and been there many times, I still have not go there. But believe it is an awesome museum.
For the 4pm-10 pm layover: go to Reston Town Center. Unless you want to spend time at one of the Airline Clubs.