Definitely the museums on the mall - the National Gallery, the Air and Space Museum, the Smithsonian, just so much cool stuff. The memorials are nice to walk around, as well - not just the Lincoln and Washington and Jefferson ones, but the Korean and Vietnam War memorials as well. And if you PM Laurie (lbaker) and Cheryl (Emily_the_Spoiled) the both work in DC, maybe you could meet a couple Pet Talkers!