Another nod to Cherry Hill although the sites are close together and their WiFi isn't that good, at least in the 1500 section. As to the using the Metro, it is fairly easy. The lines are color coded. All stations are very well marked and all cars have maps. Cherry Hill staffs a ticket booth where you can buy a token for the bus ($1.35) and tickets for the Metro. The person working in the ticket booth is well versed on the Metro. Just tell them your desired destination and they'll give you explicit directions.
I would advise against driving to Metro during the week as the parking lot is usually full with commuters. On weekends, drive (parking is free on weekends)to the Greenbelt Metro (I495 South to exit 24, one exit after you get on 495). The Greenbelt Metro is 2.7 miles from Cherry Hill and easier to get to than the College Park Metro (3.9 miles).
A trip to DC will be very hot/humid in July. Recommend one brings very comfortable shoes if planning to tour DC. Afternoon thunderstorms are a strong possibility, and for many years it seemed as though nature added to the fireworks on the 4th of July!
Cherry Hill Park is located just off the beltway (495/95) in College Park, MD and the folks there can help you with any of your tourist questions.