Arrive at Reagan National Airport in DC and follow the Metro signs (sometimes designated with just a large "M") to the Metro station. Fares vary by destination, but around 2.50USD will get you to Metro Center station, just a short walk from Hostel International.
Use the hostel's free wifi to check which food trucks will be near the National Mall that day, then take the 15 minute walk through Chinatown to Smithsonian Museum of American History, which is free, like all the Smithsonian museums in DC, and spend the morning immersed in Americana both classic and kitsch.
Head back out into the sunshine to buy lunch at your food truck of choice, like Takorean, where the Korean-influenced taco platter will fill you up for 9USD. Eat in the green space at the center of the Mall, in the shadow of the Washington Monument, and watch joggers and softball players sweating it out.
Make your way to the Tidal Basin, and rent a paddle boat for two at $14 per hour. Work off your lunch paddling around and enjoying the view of the monuments and blooming cherry trees, then stroll east across the Mall.
Stake out a spot at the Sculpture Garden at the National Gallery of Art, where the free Jazz Series will be warming up, and you can people-watch the DC locals, who love this event. Sangria and snacks are available at the nearby café.
Hop on a bus to H Street, where the quirky crowd and strong drinks at Little Miss Whiskey's Golden Dollar will warm you up as you take in the night air on their back patio.
Grab a falafel at nearby Shawafel to soak up the booze, then take the bus (1.75USD) back to the hostel and call it night.