Are you arriving on an international or domestic flight? I've been told immigration lines for non-U.S. citizens can be long at IAD. (I'm a U.S. citizen and I have never had it take me more than 15 minutes there, however.)

The 5A bus is a good, cheap option. Note that: 1) the fare is slightly higher if you don't have a smart trip card, 2) it only runs about once an hour so you could have a lengthy wait (but I find the schedule is, generally, quite accurate), and 3) for going to the museums on the National Mall, the best bet is to stay on the bus after Rossly since it goes on to L'enfant Plaza, which is about a block and a half south of the Mall, near the Hirschorn Museum (modern art) and the mall location of the Air and Space Museum. By the way, the cafeteria at the Museum of the American Indian is supposed to be notably good. Also, the Smithsonian museums (and the National Gallery of Art and the Library of Congress, which is a personal favorite) are all free to visit. Given the time limitations, those are better bets than, say, the Spy Museum or the Newseum, both of which require the better part of a day to feel like you've gotten your money's worth out of the pricey admission.

You will want to leave yourself extra time getting back to IAD. I assume, since you referred to this as a stopover that you will already be checked in for your continuing flights, but security at IAD is something of a crapshoot in terms of how long it will take.