A good thing about hiring a car is that it give a place to stash your carry-ons. You could get downtown by 4:30 or 5. The museums close at 5:30, so they're pretty much out, but you could stroll the Mall and get dinner.

A possible better plan is to see if the IAD-Udvar Hazy shuttle could get you there and back and then grab a cab to Reston Town Center where you could stroll around and get dinner in a more leisurely manner.