If you are comfortable leaving the airport and having to re-clear security, I would also vote for the Air and Space Udvar-Hazy Museum.

If you are not comfortable with the "risk" and are willing to pay for a club visit, I'd suggest the NWA World Club over the UA RCC - free wifi and booze.

Another option would be to walk and plane spot. It's not hard to see many foreign carriers and heaves at IAD, especially when riding the moon buggies between the terminals (all past security) also, if you are looking for a quite place to visit, I'd suggest the chapel on the B concorse. Finally, there are a ton of places to eat if you want to try a sampling of food.

do not leave IAD with a 3.5 hour layover.

but, you can bar hop around IAD. start at gordon beirsch at concourse D, and then make your way over to B and stop at harry's tap room, and finish yourself off at dominion brewing company! easy way to burn a few hours.

if you leave IAD to go over to udvar, the odds of you not making your flight are pretty high. security can be terrible, and transportation to/from udvar can take some time. so i would stay in the airport.