My girl Veronica has a fall edition list of great dc metro date night ideas up on her blog, and I am loving it. I especially loved her suggestion of going on a ghost tour in Old Town Alexandria. How fun! A unique idea whether you're on a first date or just out with a bunch of buds. They've got special Halloween tours, too.
If you're a bit more removed from our lovely area, Joanna Goddard has a few sweet (and cheap!) suggestions up on the Glamour Smitten blog.
photo from here