I'm a DC resident, too. Live downtown, which is a great place. If your DH can walk but not long distances, could he ride a Segway? There is at least one, if not two, Segway tour businesses in the area. You'd get to see all the sites without the physical exertion. Otherwise, the HOHO busses and an evening ride are the way to go. The monuments in the day are beautiful, but at night it's a whole other story.
Hotels are plentiful, but staying in the city is expensive. National Harbor is nice but also pricey AND really hard to get to/from DC without a car. And there are stupid rules about taxis. Alexandria is a good choice and Old Town is nice. If you bring a car to drive around the city, plan out where you're going and use Parking Panda (smart phone app) to reserve a space for popular areas. Uber has lots of cars in/around the city and you can call a regular cab with the app. Just watch the Uber pricing scheme, they go into "surge" pricing daily in some areas.
If you stay somewhere Metro accessible, rent an
ECV or motorized chair if you can. Plenty of people in both ride the Metro daily and while we may grumble about it during rush hour, we're used to seeing them and making room. Just avoid the Metro during rush hour because it's a sea of grumpy humanity anyway.
If you get in touch with your members of Congress, they can set up a private tour of the Capitol. Otherwise, you can reserve the regular tour which includes a neat movie that everyone should see once anyway. If they're in session (iffy during the summer) you could also get a Gallery pass and watch some of the pain that happens there.
If your DH is a baseball fan, check out the National's schedule and catch a game. It's a nice stadium. 8th St SW has a couple of places that make a good mojito close to the stadium and one of the bars offers shuttle service.
