We used to live in Windsor, ON and we've driven down twice. We bought the I-75 book, which we found quite useful.
The first time we split the driving up into two equal days.

Never again, that was the worst. We stopped in Dalton, GA (which is a beautiful place) and stayed at Jameson Inn (a wonderful hotel chain...we got a great deal on a room on priceline) but two days of that much driving made it seem sooooo looooooooong.
The next year we drove 15 hours (with stops) to the Georgia/Florida border. It was tiring but not much more so than the 10 (ish) hours to Dalton. We stayed somewhere cheap (Days Inn I think) in Valdosta, GA and only had 3.5 hours to go the next day (which seemed like nothing to us). We did stop in Dalton to eat (they have a Chili's there with AMAZING service) because we just love that place. We also stayed in Dalton on the way home. We hit a park until 4 pm and then drove until about 1 am and stayed at the Holiday Inn.
General Driving Tips:
Ration the fluids (for obvious reasons)
Pack plenty of snacks (if you're making good time and want to keep pushing on, stoping to eat is a big pain so if you have food in the car...you can stretch out the meal stops to more convenient times or just eliminate them totally)
Multi-task the stops (eg if someone has to pee, make everyone get out and go, then walk around stretching for a few minutes, eat if it's near a mealtime and then gas up and get back on the road)
Bring LOTS of things to do (especially for kids, we give them a backpack each and tell them to fill it with stuffies, figurines, colouring items, books, movies, games and electronic devices. And mom and dad bring things for themselves as well. Long drives are boring!)
Make sure EVERYONE gets out to stretch at EVERY stop (it helps keep the crankies to a minimum...weird, I know but it works).
Expect unexpected things to happen. They will. Traffic, the weather, the car, the health and mood of the family members...it could be anything and it could throw you off track completely.
Bring a barf bucket. (Our kids don't EVER get motion sickness but for some reason, DD's nerves get to her when we're headed to Disney (and ONLY Disney) and she gets sick two to three hours into the drive. We flew from Detroit the third time and she almost got sick on the way to the airport!! Never when we've travelled anywhere else though.

Some recepticle that can be used when you can't pull over avoids a potential mess and even if you think you don't need it (and DON'T need it) it's just not worth risking.)
Make your progress a game. (The time seems to pass faster and the mood remains more cheerful if you greet each new state as you cross over the state lines and look forward to the city or landmark that's only XX hours/minutes away from your current location.
Wish your friends luck for me!!

And I hope they have a magical time.
