D.C. is a deceptively small city, as the District population hides the fact that it is at the heart of one of the country's biggest, most densely populated metro areas—more than five million people live in the immediate D.C. area. As you might expect, there is a lot to see in the suburbs, including some of the capital's principal attractions, like Arlington Cemetery, and all three airports, for that matter. The suburbs also have a lot of hotels just across the District line near Metro stops, catering to travelers looking to evade the dreaded D.C. hotel tax.
Northern Virgina, or more obnoxiously NOVA, is best reached via Metro. Nearly everything of interest is just an easy walk away from a Metro station. Driving is a terrible headache, as Arlington and Alexandria have the worst traffic jams in the metro area, and the street layout is even more labyrinthine than that of the city itself. With the sole exception of the Arlington Cemetery stop on the Blue Line, all the main Arlington stations are on the Orange Line. If in Georgetown, the #38B bus leads right across the Key Bridge into Rosslyn, but it's not a bad walk if you'd rather skip the bus fare.
For Alexandria, the King St Metro stop on the Blue/Yellow Line is much more convenient than driving to Old Town. Annandale, on the other hand, would be a great foodie destination if it weren't so hard to reach. The only way out there is by car, where ample parking awaits. To get there, take the Little River Tnpk exit heading east from the Beltway, west from 395.
Arlington once comprised most of D.C.'s Southwest, before retroceding into the state of Virginia. This is the one "suburb" that most visitors to the city do see, as it is home to National Airport, the Pentagon, and Arlington National Cemetery. It's also just a highly urbanized center, with lots of restaurants, bars, hotels, etc.
- Teddy Roosevelt Island and the memorial to the President therein are just south of the Key Bridge on the Mount Vernon Trail. See the overview article for detailed information.
Old Town Alexandria rivals Georgetown for sheer historical charm. Alexandria historically was Georgetown's rival port on the Potomac, vying for supremacy in the lucrative Mid-Atlantic trade, particularly tobacco and slaves. Alexandria's fortunes started to decline during the early nineteenth century, especially following the construction of the Chesapeake & Ohio Canal north of the Potomac, which benefited only Georgetown. Amid continuing complaints that the federal government was neglecting investment in Alexandria, and over sharp fears that the District would ban the slave trade, Alexandria led the retrocession of the southern portion of the District (what is now Arlington County and the City of Alexandria) into Virginia.
Alexandria is one of the nicest places in the Metro Area for a stroll, a visit to the Torpedo Factory, and a night of fine dining and drinks. Since it's just off the Metro Yellow Line, it's an easy and rewarding little trip out from the city.
This neighborhood is hard to reach, but some of the best Korean food outside the motherland is served up in Koreatown's many restaurants. It's also a late late night hotspot, with Korean BBQ served 'round the clock every day.
Bethesda, Silver Spring, and Wheaton are extremely easy to visit by the Metro Red Line. Each of their metro stations is located right by the downtown, and you'll be able to reach everything listed below in less than a ten-minute walk.
Kensington is harder to get to via public transport, and is better visited with a car. The Mormon Temple is tricky to find (although it is easy to see!). Take Beach Dr east from Connecticut (just north of the Connecticut Ave Beltway exit), then turn left on Stonybrook Dr. For the carless, hop on RideOn Bus 5  from the Silver Spring metro stop, which will take you straight to Antique Row and Continental's, and then on to the Mormon Temple (get off at Stonybrook Dr and walk about ten minutes south).
Three main roads head north into these Maryland suburbs. From east to west: Georgia, Connecticut, and Wisconsin Ave. Georgia runs north through Silver Spring and then Wheaton, Connecticut runs through Kensington, and Wisconsin runs through Bethesda. Traveling east-west between the suburbs is harder. University Blvd (north on Connecticut from Kensington) connects Kensington to Wheaton, but otherwise you'll be best off taking the Beltway (I-495), which has dedicated exits for each neighborhood (take the Connecticut Ave southbound exit, then turn west on the East West Hwy for Bethesda).
Bethesda is suburban destination number one in Maryland for wining and dining, a much more family friendly part of town than anywhere in D.C., and it's right on the Metro's Red Line, largely along and near Wisconsin Ave.
Silver Spring is also easy to reach, as it is just across the District border on Georgia Ave, with a Metro Red Line stop right in the center of things. It's a good place to hang out, but the main reason to come is for a movie at the AFI.
The Mormon Temple is by far and away Kensington's most significant attraction. Most only see it from the Beltway, where it rises high above the treeline, looking similar enough to the Emerald Palace in the Wizard of Oz to inspire a local graffiti artist to paint "Surrender Dorothy" on the overpass on the Outer Loop of the Beltway. (The temple is actually white, though.) It's not possible to enter the Mormon Temple itself if you are not Mormon, but the grounds are pretty, and there is a Visitor Center:
'Tis the season to visit the Mormon Temple come Christmas time. The light display is truly spectacular, especially given that the grounds and temple are impressive enough to merit a visit without the display. The light display runs throughout Advent, and is absolutely worth seeking out from D.C. if you are in the area this time of the year. In addition to the light display, the Visitor Center shows its best face, with more than a dozen large, decorated Christmas trees, international nativity scenes, and a live outdoor nativity scene. It's a great place for all ages, and is certain to stir a little of the Christmas spirit.
The Old Town of Kensington is a major shopping destination in the D.C. area for antiques, with a large cluster of antique shops along Antique Row.  The strip is charming and full of interesting browsing opportunities, from furniture to bric-a-brac to books. To get there, take a right off of Connecticut Ave (coming from D.C.) after the Knowles Ave intersection onto East Howard Ave. On street parking, as well as shop parking is usually no problem.
Very few visitors know of Wheaton, much less make it out here, but it's easy to reach on the Metro Red Line (north of Silver Spring), and has D.C.'s best ethnic dining scene, period, amidst what is probably the most diverse neighborhood for 100 miles in any direction. The Washington City Paper's 2009 critics' choice for best Thai in the whole metro area went to both Nava and Ruan. And it has the legendary Peruvian pollo a la brasa joint: El Pollo Rico. Coming out of the Metro station, admire that escalator—it's the longest in the Western Hemisphere, taking over four minutes to ascend!