Unless you have a vehicle and want to drive into DC, you will need to take the Metro into DC. Last time I was at Wyndham NH, they provided a shuttle to the closest metro stop (IIRC it was near the Wyndham Alexandria). Might want to ask about the shuttle if they are still running it. Have not been back there since Marriott Pulse opened in DC.
I'll make some changes to frank808's post.
Even if you have a vehicle YOU DO NOT WANT TO DRIVE INTO DC.
The free shuttle is long gone. Your easiest way to the District is the Metrobus, Route NH-2. It runs from National Harbor to the King Street Metro Station. Take the Yellow Line train into the District -- L'Enfant Plaza, Archives, or Gallery Place are your most likely stops in DC. Change to the Blue Line at L'Enfant Plaza and the Red Line at Gallery Place.
The NH-2 bus stop in National Harbor is a very short walk up the hill. Out the front door, go across the street and go left. You'll see the bus stop on the cross street. You'll need SmartTrip cards, one for each person. Get SmartTrip cards here:
https://smartrip.wmata.com/Storefront/ShoppingCart/AddAndPurchase/6745467
They cost $10 and are pre-loaded with $8. That'll get you started. Add value at machines in Metrorail stations. Take the NH-2 bus back to National Harbor from the King St. Metro Station.
The route and schedule from NH to King Street:
https://moovitapp.com/index/en/public_transit-line-NH2-Washington_DCBaltimore-142-1741632-24148886-1
And the return to NH:
https://moovitapp.com/index/en/public_transit-line-NH2-Washington_DCBaltimore-142-1741632-24148886-0
I've attached a National Harbor map and the MetroRail route map. The Wyndham facility is labeled QQ. The bus stop is on St. George and is marked with a tiny bus symbol.
Restaurants in National Harbor are pretty good but tend to be a bit expensive. There aren't really any "family restaurants." If the timing works out for you to spend some time in Alexandria, King Street teems with local and chain restaurants and boutique shops. From the King Street Metro Station to the Potomac River is a little over a mile. Fortunately, there is a free "trolley that runs the length of King Street. Info here:
https://visitalexandria.com/plan/king-street-trolley/
A caution: Distances between museums and other attractions on the Mall are deceptively long. You will not be able to do "all the museums." There are too many and you'll not have a good time if you try. Some may require reservations. The National Crime Museum is one that I think you'll have a hard time just walking in. Do some research on the places you want to visit and pick you top half dozen or so.
Take a day to go to Mount Vernon at least. It's a short drive from NH. The Marine Corps Museum is down I-95 a ways at Quantico. Thomas Jefferson's Monticello is about 2-½ hours away in Charlottesville. The National Air and Space Museum on the Mall is a must (for most people anyway) and there's an annex called the Udvar-Hazy Center out near Dulles Airport.
The Washington Nationals are out of town so mercifully, you won't be tempted to go to a ball game. There was not much good baseball played in DC this year, at least not by the Nationals. (I was a half-season ticketholder for years. Gave it up before the 2020 season, when you couldn't go to a game anyway!)
Have an enjoyable stay! Late September is one of the best times to visit the National Capital Region. It should be warm but without the oppressive humidity that we get in mid-summer.
Edit: We live about twenty minutes from National Harbor and stay there now and then, most recently the third week in June. We moved here in 1985.
wmata.com
www.cityexperiences.com
moovitapp.com
