14-yo girls may well enjoy the spy museum, although parts may have a little too much in terms of reading (captions, etc near the exhibits). The basic information is quite interesting, plus the addition of things that might appeal to teens like the James Bond car, etc.

I always like the Old Post Office tower, free to access, no lines except maybe 2 minutes to wait on the elevator. Not the views of the Washington Monument, but just as good on most days when the pollution obscures the WM's views farther out. The Old Post Office is also open up top, which I like.

For shopping, depending on the personalities of the teens, you may go for Adams Morgan (funkier spots) or Georgetown. The zoo can be great, with a nice lunch in one of the restaurants in the area (Woodley Park/Cleveland Park) and, since it's part of the Smithsonian, it's all free -good stuff if they're on a babysitting budget!

My favorite "typical" Washington monuments (ie statues) to see are the Einstein statue near the National Science Foundation and the Roosevelt Memorial. Young teens might also be interested in researching the name of an acquaintance/relative who didn't make it home from Vietnam and looking up his name on the wall (although this is far more of a lesson than an enjoyable/sightseeing venture).

Enjoy your trip.