We have plenty of time between now and then. I will craft you a dining itinerary for DC as we learn more about yours.
From a culinary standpoint, Philadelphia is like my home away from home. An upgrade from Baltimore, and beats the crap out of what I've got in my own back yard. I will help out there, too, with a very respectful nod to the locals.
A quick comment about Philadelphia sandwiches, cheesesteaks and otherwise:
Pat's/Geno's: agreed - don't bother. Time has passed them by, and they are more of a tourist trap than anything $-worthy at this point.
Very sorry to have to say the same about Tony Luke's. Years ago, that was the first experience I had outside of Pat's/Geno's that opened my mind up to the sandwich glory Philly has to offer. Unfortunately, I think TL's is fast approaching the same graveyard.
Jim's is a step above Pat's/Geno's/TL's. I would settle for a steak from Jim's. Dalessandro's near Manyunk is my personal favorite.
http://dalessandros.com/
John's Roast Pork is very good. So is DiNic's.
Reading Terminal is a very cool place to graze at lunch.
Do any of you old school Philly guys remember Shank and Evelyn's off 9th St.? They've moved to Rittenhouse/Center City, and I've heard it's different now, but back in the day, that was the only place to go for a Roast Pork/Beef Sandwich with real aged provolone, peppers, onions and broc rabe. F'ing amazing.
Anyone want to talk about Tacconelli's Pizza? I could wax poetic, but a native will probably do it better.
...but when I really want a sandwich, a really great sandwich, the place at the top of my list is Chef Peter McAndrews' Paesano's
http://www.paesanosphillystyle.com/index.html
There's a newer one in the 9th St. Italian Market, but go to the original dirty hole-in-the-wall on Girard, near Fishtown.
I'm a huge Vetri/Michaud fan/disciple. I have learned a s-ton from those guys, and respect the heck out of them. They recently partnered with Urban Outfitters. I don't have a problem with that, now that the hubbub has died down. They're still a great bet. I'm a bigger fan of Restaurant Vetri and Osteria over their other shops, but they're all good in the niches they occupy.
I'm pretty tight with DiBruno Brothers. If you're looking to do something food-related in Philadelphia, the 9th Street store is a must-see, and so is the newer location in Center City. Right around the corner from Center City DiBruno's is an absolutely fantastic wine/beer bar that serves some excellent food too, called Tria. They now have multiple locations, but I like the original, near DiBruno's, the best.
More on Philly, and plenty on DC, to follow...
ETA: Brandywine Valley area is no joke. Well worth checking out.
The Frank Lloyd Wright home, Fallingwater, is a very cool thing to check out, if you're into architecture:
http://www.fallingwater.org/