Luray Caverns (in Luray Virginia), you should go. Especially if you have kids (bring them along of course). Luray Caverns is one of the most impressive underground cave systems in the country, and certainly the best one within a days drive of Maryland. It is less than two hours away from Bethesda so you can easily make it a day trip.
This isn't a place you go back to year after year. It doesn't change much. However, going there every 20 years or so is well worth the effort. If you have already been there, you should consider going to Skyline Caverns too. I don't think it is quite as awe inspiring, however, it does have some types of rock formations that Luray doesn't have, so if you like Luray, you'll probably like skyline too. I have some more pictures of Luray Caverns if you want to see them.