Things for out of town guests to do who have done all the normal DC things?

My family comes to visit from abroad for several weeks every year over the holidays, and again in the summer. They're here now and we're realizing that we're running short of ideas for things to do during the winter. Since we moved here 6 years ago, they've done all the museums and monuments; have walked Georgetown, Alexandria, various DC neighborhoods, and the Wharf etc. many times; and have gone to Harper's Ferry, Annapolis, Richmond, Frederick, and Fredericksburg. I should note that they have limited English. Also, my parents are a bit older and not outdoorsy people, so things like hiking or biking are not for them. We spend plenty of time doing movies, games, and cooking and whatnot, so I am looking for the occasional activity or excursion outside the house. I come to you, humbly, Redditors of NoVA, what great ideas do you have up your sleeves?