Sorry -
I should have said "Point Lookout" - 50 miles SE of DC.
It's the southern tip of Maryland on the Chesapeake.
If you want to start on the actual Atlantic, Cape Henlopen is a good choice.
Easiest to get to from Philly airport and Jersey Transit bus, then ferry.
It is tough getting across the Chesapeake bridge.
Maryland DOT could give a sh1t about cyclists.
Or you can take passenger ferries from Crisfield to Smith Island to Point Lookout.
South Dakota is a good choice. Hwy 12 in far northern Nebraska is really nice.
Then Hwy 44 in SD. If you are willing to do hardpack - I'd take the full loop in Badlands.
Remember that SD is really hot in mid summer - esp. the Badlands. Ride at dawn.
Plus the Black Hills are an extra treat in the Great Plains - cool oasis.
In my humble opinion - skip Mt. Rushmore, though.
J