The trip to Ocean Isle Beach (a little bit north of Myrtle Beach, in NC), has taken us about 9 hours. We did it at 10 weeks and 14 months. 10 weeks was easy. 14 months actually went ok--I think easier on him than me.
The last time was with a rear facing convertible carseat. I think the seats are all pretty comfy, but the rear-facing is hard. I found that it was much easier to be in the back seat with him. We are looking forward to turning that seat around before our next trip, so we can see him and talk to him from up front.
Peekaboo, songs, little toys...the funniest little things are sometimes the most successful in keeping the little ones entertained.