Washington DC

My fiancé and I are planning a honeymoon to DC for this upcoming July. I put in a request with my senator to hopefully score a tour (white house, capitol, etc.) Besides that, we want to explore the Smithsonian, take a quick train ride to catch a tour at Mt. Vernon, and we would love to tour the National Cathedral. Any advice, tips, do's/don'ts? What are some must-have local food or other things to see? We are from Iowa and we do not get to travel much so we would love to go above and beyond, as well as for the honeymoon sake. We love history and exploring cities, hence why we chose DC because it is up our alley. Thank you in advance for any advice or recommendations!