New York; Gray Line Cruise, maybe up to an Indian Casino (for the countryside as much as anything.)
Washington: The Mall, the Air & Space Annex at Dulles (or the main building on the Mall).
Texas: a gun show is always fun. A rodeo if possible.
California, some sort of beach culture place as ours differs so much from theirs.
Near DC, I happen to be a fan of Annapolis, but then, I’m a sailor. Still, Annapolis is a neat city on its own. If you were coming in a warmer month than November, I’d invite you to spend a day on the bay sailing with us, but the weather can be iffy that time of year.
Solomons Island in Calvert County or St. Michael’s on the eastern shore have maritime museums that feature all kinds of interesting tidbits about the lives of the watermen on the bay and the early history of this area.
Baltimore - Fort McHenry, Fells Point, The National Aquarium in the Inner Harbor are always favorites with our visitors.
Fredericksburg, Virginia and thereabouts for a lot of Civil War sites and such.
Absolutely - Our Fair City can’t be beat as a tourist spot. 
The National Cathedral and the Library of Congress are particularly worth seeing, IMHO.
A few stops in CA and TX? I’m guessing that you mean that they fly into CA. The next stop flys them into TX? Then onto Philly where you are at?
My Wife and I spent four days touring the Smithsonion museums (there are about 10) and monuments in DC. We did it fast. Saw most everything on a sort of Chevy Chase Grand Canyon type of tour. No dilly dally.
A month? hmm… I would maybe consider driving back from the East Coast to Memphis. Through the Appellations. From there, grab a flight to Denver. Do the Rocky mountains on a drive through Colorado. Oh… November. Can get a get dicy if you are not used to snow. They could fly into Colorado Springs (a real nice smaller airport) and take a more southern route. This would set them up to see the Grand Canyon if they wish. Utah also has some amazing sites.
Vegas? It sort of has to be seen, but it’s not my bag.
It really depends on what kind of vacation they want. If they want to try to see what America is like in a month, they are going to have to really, really move it.
A trip through Amish country might be fun, though it’s a bit of a detour. Any major art museum in NYC, of course, followed by a pizza at Gennaro Lombardi’s.
Don’t drive down the east coast; take Amtrak. Your friends’ memories of the USA should not cluster around rest stops on the New Jersey Turnpike.
Stake out some great jazz and blues nightclubs; that’s the best face America has to put forward to the rest of the world. I don’t know the music scenes in Boston or NY, but Blues Alley in DC and Fat Matt’s in Atlanta come quickly to mind.
A couple of shows in Branson, Missouri might be “quintessentially American,” but that’s one day you’ll never get back. Austin TX is a much hipper musical showcase. So is Nashville, but only just.
Broadway. And Junior’s Cheesecake on 45th Street.
I have taken “long weekend” (Easter Break) road trips with Exchange students to Niagara Falls, Bar Harbor Maine, and the Smokey Mountains in Tennesee at seperate times departing here from Ohio. My foreign friends and I really enjoyed the trips, even though it was a lot of driving. And the natural beauty and wonder of these areas are entirely unique to the U.S. (There is also some of the best white water rafting in the US near the Smokies.)
Considering your location, you are even closer and more central to these destinations, (possibly, with the exception of the smokies). and could easily drive to any of these areas in a days time.
Lobster and corn on the cob boiled in Atlantic Seawater from an ancient Lobster shack by the shore near Bar Harbor and Acadia National Park is a pretty damn, singularly, incredible experience as well.
Maybe a target range. If they don’t fire a gun at least once, their pals in Australia won’t believe they were really here.
Another thing about The Great Smoky Mountains National Park is the wildlife. Both times I’ve been there I’ve been able to observe Black Bears in their natural habitat for extended periods and was even able to get fairly close to one (not necessarily advisable, but an awesome experience, nevertheless.). As I mentioned, there is also canoeing, kayaking, and white water rafting rentals and tours. There is also an aboriginal prescence with the Cherokee Reservation, nearby.
Deadwood, Mt. Rushmore, the Crazy Horse Monument and Wall Drug (all in South Dakota) and the site of Custer’s last stand in Big Horn County, Montana (south central Montana.) Then continue on to Yellowstone, Reno and Lake Tahoe, Yosemite, and San Francisco.
You could also take a trip up to Lake Champlain and the Adirondacks and go on a Champ hunt. You might have a sighting, who knows?
If not, I understand it’s beautiful country anyways.
High school game would be cheaper and more accessible. Especially in the American south, college games might be sold out (effectively) already. My other comment is that we would get people from Europe, etc. coming to my church, and they would think they were athletic, and then we’d all find out that unless they played a lot of cricket, they had absolutely no skills at baseball/softball. So just be aware that you need to be in a very low key game, and be prepared to really suck, IME. Rugby, handball, football, hockey do not have the batting motion or throwing motion, and many guys in our pickup games who were being eyed pointedly by the female population pretty much got pwned.
November is pretty mild in Tennesee, almost like fall. Probably few bears though. Or possibly at the cusp of hibernation. Didn’t see the November in there, take them south. Get some cheesesteaks and go to the Mutter.
Probably because November is in fall.
Truly, but it’s like Winter here to the North. November is winter to my climes. We’re a bit behind on the times.
Come up to Montana and say that. 
If you’re living in the Great Black Swamp, i.e., northwest Ohio and a bit of northeast Indiana, November is hardly winter. The average daily low in Toledo doesn’t reach 32°F until November 30.
Average is different than the reality. Winter starts about Nov. 15
Lake effect.