This is a terrific idea and your boys will have lifetime memories!
It's also good to pinpoint attractions on a map as the suggestions start rolling in - perhaps not just one U.S. map but instead, get a good road atlas (WalMart has one with big print so small roads are easy to see), tear out the states you'll be crossing and tape them all together to get the big picture. The boys will love helping and it will be a good geography lesson for them.
You've already gotten some good ideas from others and when you get a better Google map up for us we'll all be able to see what you have planned. Warning though - you're going to get 'information overload'. :)
