the foot rest is the way to go. I have been traveling down to FL with my grandson since he was 1 and he needs to have a place to rest his feet otherwise they seem to cut off circulation at the back of the knees and he has trouble walking for a few minutes. We always use a cooler, it fits nicely behind the front seat and has easy access when we are thirsty.
I also always bring the video player. On our last trip I taped all his favorite shows and he played them over and over.(19 hours of Dora, diego and the Wonder Pets...) We are going this July and I will set up his WII to the player. I don't have a CD player, just the old fashioned VCR but it works...
Also, lots of juice and snacks, he seems to get hungrier when he is in the car than when he is outside playing. Make sure to take rest stops. when my boys were growing up and I took them the rule was bathroom and running when I stop to get gas, which was not very often. My grandson won't wait that long, he has to stop every 2-3 hours.
I am going to look for one of those pilows because since he moved up to a non reclining car seat his head lobs when he falls asleep and I get nervous about his breathing. My son said if he couldnt' breath he would wake up but I am a grandma, we are supposed to worry.