This is a classic but we love the national mall. Sabrina's favorite attraction is the Lincoln Memorial (Great sunsets!) and Carmina's favorite is the Washington monument (Hot tip: Bring your longboard or an electric scooter to zoom around the base of the obelisk).
Also while you're in DC? The National History museum. Carmina could stare at the Giant Mammoth and walk around the ocean exhibit for hours (All save for the airborne whale). Sabrina enjoys the mammal exhibits and the gemstone exhibits. She wishes Carmina would buy her the Hope Diamond one day.
The beach! We LOVE east coast beaches. Even though the water might not be warm enough for the tropical paradise of your dreams, we highly recommend going for walks, shell hunting, or swimming in the waves. Bonus: Chincoteague Island (2.5 hours east) hosts the infamous wild horses who roam the beaches and marshland. Or you can head to Rehoboth Beach (1.5 hours east) to see where Sabrina proposed to Carmina!