I know this seems random, but there is a restaurant/cafe called Cups in the basement between Dirksen and Russell on the Senate side of the Hill that makes great panninis. There is one with tomato, pesto, and mozzarella on a sourdough bread. It's quite good (plus Cups makes a good mocha.)
Thank you both! We'll be staying at the Washington Plaza, and I think I heard there is not much to eat around there. So I am envisioning having to take the metro from the conference center to eat or schedule time when touring the sites. Please correct me if this is not true.