I'd suggest you spend as much of your time as possible in DC and Maryland, and use the DC Metro system to get around. There are many fine restaurants, hotels, and shopping areas quite near the various Metro stations.

Personally, I'd suggest you stop at Dupont Circle, eat a really good meal, and go window-shopping. Get an all-day pass and travel all over (the postal museum right at Union Station is quite fun and cute) after 9:30 am on weekdays.

If the relatives really want to go out, a trip into DC for dinner is entirely reasonable.