We just went east from Michigan in August. We drove through 6 states, including Pennsylvania and Maryland. There are some grades in Pennsylvania but overall, it was not as bad as driving through Tennessee.
Have a great time. We've had to reschedule our trip to DC for 2 consecutive years now. Eventually, we'll get there!
As far as other destinations go that kids may enjoy; my kids have enjoyed just about every location we've gone to. Smoky Mountains, Chattanooga, Mammoth Caves, Cuyahoga National Park (this one is very close to home and is great for a long weekend. Bring bikes), NYC, Assateague Island (a HUGE favorite of ours), Williamsburg, Baltimore, Philly.