Udvar-Hazy Center is 10-15 minutes and about a $10-12 cab ride from the airport. Since you shouldn't have any need to stop at baggage claim when you get off the UA
flight, you should be able to be there within 30 minutes of landing.
Admission there is free, so spend about 2-2.5 hours looking around there and head back to the airport. Keep in mind that security at IAD
can take a while to get through in the early evening, so leave plenty of time for that.
I believe there's also a mall around the airport that you could probably cab to if you were inclined to shop.