I've also heard good things about Vino Volo. As violist mentioned, its down near the low C gates. The problem is that LH and UA use those gates for quite a few European departures between 4:00 and 7:00 pm, which can mean crowds. When I was at IAD about 9 days ago for a 6:00 pm flight to ORD (that happened to be on a 747) departing from C3, there was a pretty substantial line of people waiting to get a seat at Vino Volo.

The one restaurant/bar outside of security is Harry's Tap Room. I've not eaten there so can't say much about it. I have eaten at the other branch of Harry's in Clarendon and wasn't that impressed -- it seemed like a slightly higher priced TGI Friday's.

As gre mentioned, you could also head to Reston or Herndon for dinner/drinks. While Herndon probably has more character, you might find a greater variety around Reston Town Center.

You might also want to think about something in the Tysons Corner area. You'd be able to use the Dulles Access Road all the way to Tysons, meaning virtually no traffic (until you exit at Tysons) and no tolls (if you exit at Reston or Herndon, when you get back on 267, you'll be on the toll road - cost from there to I-66 or I-495 should be $1.50 or less). There are some pretty decent dining spots/watering holes in the area around Chain Bridge Rd. and Route 7, and easy access from there both to I-66 and the beltway.