Surprised people haven't talked about the food yet?! What great flavors! A few of my favorites are in no specific order...
Smashburger for sliders & pizza (1st meal I ever had in DC)
www.matchboxchinatown.com
Thaiphoon for Thai & large yummy adult beverages
Www.thaiphoon.com
Good Stuff Eatery or We the Pizza (they are next to each other). Owned by Top Chef contestant & Food Network Star Spike. Lots of great burgers, fries, milkshakes & pizzas that are done in amazing ways. Not commercialized, it's where the locals go but has the extra fun of being celebrity owned (I took my picture with him)
www.goodstuffeatery.com &
www.wethepizza.com
For a real local place... Eastern Market. If you can make it on Saturday morning, they serve amazing blueberry buckwheat pancakes. The line will be crazy but it is fun. After you eat there are local shops to shop at.
Www.easternmarket-dc.org
Lastly I wouldn't be a good friend if I didn't mention The Union Pub (close to Eastern Market, which are in The Capital Hill Neighborhood). My "BFF" is a manager there & I've had the brunch... So good! If you meet "Paige" tell her Sleepy Sarah plugged her on a Disney message board... She'll figure it out

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Www.unionpubdc.com
DC is a great trip with so many things to explore for free & the Metro makes it easy to get around.
My favorite spot is taking the elevator to the top of the Washington Monument. You have to get there early for tickets but they are cheap & will reserve you a spot for later in the day (like FP?!). The views are breathtaking & whenever you see the Washington Monument you can say "I've been to the top of that"