#1- If you can afford it, buy a Britax seat before you go. We have taken my son from Southern California to Northern Oregon/ Southern Washington twice now. He has NEVER complained about the long trip. We also travel from Anaheim area to Temecula area about once a month. He LOVES is car seat! They have a lot of convertibles that go to 65 lbs with a 5 point harness. (Many convertibles rated to 65 lbs expect you to use the regular shoulder strap seatbelt after 40-45 lbs.) (Babies"R"Us has The Britax® Marathon Convertible Car Seat - New Denim for $250 right now.) $250 may seem like a lot when you're about to take a road trip, but it is a long term investment in your son's safety as well. Britax is one of the best rated brands available in the U.S. It is, and ALWAYS will be, my top pick for car seat brands.
#2- Books, bubbles (the tiny one's), and a NEW (at least to him) interactive toy. We went to Children's Orchard before our last trip and bought one of those "pretend to drive" toys for about $5. We also took turns sitting next to him when he was bored. We also took a container with a handle that all the loose toys would fit in. I think we picked it up in the craft section at Wal-Mart for $6 or $8. We put his special Teddy Bear (he named it Buddy), a coloring book, crayons, and a few misc. toys in it. It was very helpful during the nights in the hotels.
#3- Pack at least 2-3 days worth of CAR friendly snacks, and keep them where the non-driving adult can access them at any moment with out pulling over. If the car stops, he will want to get OUT. Once he's out, it will be about 45 minutes to an hour before you will get him back in. Goldfish, crackers, apples (w/ a wedger), cheese sticks in the cooler with juice boxes and mini waters... You only need enough for a few days, as (amazingly enough) grocery stores are available nation wide. :) These snacks should be sufficient for you and hubby as well.
#4- Don't over pack the diapers. Just be sure you have enough for 2-3 days at a time. How ever long the small pack lasts. Diapers are about the same price no matter where you go, so you don't need to pack a 14 day supply.
#5- Last but not least, pull over and let him, you, and hubby stretch after his nap and about every 4 waking hours. This is good for you guy's as well as your son. Sitting for too long will cause all of you to become restless at night, which means NO ONE gets to sleep well. When you know the 4 hour mark is getting close, start looking for the rest stop. They're free, have plenty of grass (usually), and restrooms. (I also recommend stocking up on some toilet liners and packing a roll of TP, just in case.) We also had a bigger bubble that we used at these stops and a blanket so we could "picnic" if there were no benches, or only gross ones.
Sorry it's so long, but I hope it helped. Have a great trip and take plenty of pictures. If you use a digital camera you can take it to a Walgreens and they'll save the pictures to a cd for about $3. I believe Wal-mart and Target do it too, but I don't know how much they charge. That way you can dump the pictures on the camera mid trip if you need too. Hope you have plenty of AWESOME memories from this experience~ J.