What is a "MUST SEE" in Washington, DC?

I want to experience Washington DC to the fullest this spring break. Are there any PLACES TO EAT that I should definitely try? Any cool places famous for their food? I'm open to great restaurants or even little joints. Where should I go to experience the best NIGHTLIFE? (Preferably for ages 18+) What monuments are best experienced AT NIGHT? What museums, other than the Smithsonian museums, should I not miss out on? Just basically, where should I go to have the best experience in Washington DC. Hope someone can help me! :) You'll win "Best Answer" if you answer every part of this question with good recommendations!