My son hates road trips! Any advice to make it more bearable?

My son is 19 months now and we recently went to visit my family that lives 3 1/2 hours away. Well at first he was fine then the last hour and a half he started crying. He wanted to get out of the car seat, nothing calmed him down. Same thing on the trip back. Well we have a trip coming up that is an 18 hour drive, so we are going to be stopping alot to let him walk around as well as get a hotel etc... I was just wondering if anyone had that same problem with their little ones and if so, how do you keep them happy. I know in time he will grow out it but this trip is coming up fast and I do not want to be miserable the whole way down and then redo it when we come back a week and a half later. Any advice would be helpful? Thanks!