M.R.
We go to the theatre a lot so I like that idea too, as others have mentioned; if they are willing to go into DC for theatre, the Shakespeare Theatre has two terrific spaces that do Shakespeare and MUCH more, and Arena Stage specializes in American drama and the big musicals.
But you do note that they are backpackers and travelers. If they are into the outdoors, consider a gift card for an outdoors outfitter store like REI (large chain so there are several around) or Hudson Trail Outfitters. They can get a wide variety of backpacking gear there.
Another option would be a certificate for an outdoor adventure -- if they are outdoorsy but haven't done kayaking, for instance, you could get them a half-day of kayaking instruction. LL Bean actually does this and runs excursions out of Tysons Corner Mall's LL Bean store, going to local lakes. Be aware that these run only on certain days so it's the kind of gift where I'd just ask the recipients if they want to do it before I got it. If they are experienced kayakers already this would be too basic for them, though.
There are also indoor rock climbing facilities around Northern Virginia and if they'd like that , check those out online.
If they are in Annandale they are not far from the new, hot "Mosaic District" of restaurants, movie theatre etc. If they like a nice meal out, go to the Mosaic web site and research the restaurants there. Maybe combine with a gift card for Mosaic's Angelika Film Center for movies, if they prefer movies to theatre.