Airports - Ronald Reagan National (DCA) and Washington-Dulles International (IAD). A larger part of which of these two airports you arrive on depends on the carrier you are traveling and whether you will be staying inside or outside the city. Dulles is about 40 minutes outside downtown it is a large airport and some terminals require a trip in a trolley, so leave extra time. My biggest complaint is no rail service directly to the airport.
Train - By now everyone knows how much I love trains, and DC's rail service was no exception. Day passes are the best option. You will see vagrants but the stations and the trains themselves are clean and well maintained.
Auto - Construction Construction Construction. Traffic Traffic Traffic. With some hundred thousands of visitors foreign and domestic arriving at the city daily be prepared for some snarls especially on the beltway and downtown.

Washington Mall - You could easily spend the better part of a day or at least all of an afternoon visiting the monuments. A fit person can walk the extremities of the mall without a problem. Some of the distances are long though (a mile or so between monuments). Bring someone to talk to.
Kennedy Center - Parking is very convenient at the center. Get your tickets in advance and go early. Visit the the Roof Terrace Restaurant for hors d'oeuvrerie and atmosphere. People are very friendly and conversations come easily. Plus you can't beat the views for a romantic evening.
Great Falls National Park - Great place to unwind about an hours drive outside of DC on the Northern boarder of Virginia. Sit along the Potomac River and have a picnic. Very easy trails to navigate with historic sites of an old canal and lock system. Special attractions are rock climbing, bike trails, and kayaking.