What people seem to like to see the most from the air is ...stuff on the ground. So as long as turbulence isn't bad, I'd suggest low level sightseeing. Out of BWI to the eastern shore of the chesapeake, north to the susquehana then up the susquehana to Lancaster for lunch and an opportunity to switch seats for the return. I always keep someone's first flight short.
Show them around the aircraft and explain how things work beforehand. Explain how long the trip will be beforehand as well. Lots of nervous flyers are more comfortable if they know how long they have to endure it. Keep the doors/windows open as long as possible. A hot stuffy cockpit will get some people sick quicker than turbulance.
Enjoy, if you're relaxed and happy, they will be too.