Here is some advice on road trips with babies!
* A friend recommended putting the baby in overnight diapers to prevent rashes from lots of sitting and keep the baby from complaining between stops. We ended up not doing this because Kit doesn't really complain about wet/dirty diapers; we just stopped every two hours to give them a break from the car seat (which is very important to do) and changed them then. That said, they did start to get a bit of diaper rash redness by the end of the trip, so if you don't use super-absorbent diapers, I recommend applying diaper rash ointment proactively/preventively.
* If possible, stop at restaurants rather than highway rest stops. It's so much easier and nicer to change the baby in a restaurant bathroom than in a noisy, crowded rest stop bathroom where a dozen high-velocity hand dryers make a horrible noise that makes the baby scream, and restaurant food is better and tastier than rest stop fast food. Second-best option for a quick change: stop at a Babies R Us or similar baby supply store, which is guaranteed to have a well-appointed changing room.
* Expect each stop to somehow take at least 30 minutes even if all you're doing is changing the baby and giving them a little wriggle time. (Pack a picnic blanket you can lay out on a table or a bench or that little strip of sorry grass next to the parking lot.)
* Bring a few extra layers of clothing for the baby so you don't feel any urge to drape a blanket over the car seat even if you're cranking up the air conditioning in the car.
* If your kid isn't yet weaned, you may want a policy of only feeding liquids in the car so you don't have any concerns about choking hazards. Obviously you should NEVER EVER breastfeed in the car. Apparently this is a thing people do? Don't do it. Bottle-feed in motion, or breastfeed at a rest stop.
* Pack your regular diaper bag with everything you'd need for a day trip and have it handy for rest stops. If someone's sitting in the back seat with the baby (HIGHLY RECOMMENDED) they can also dip into it for toys, pacifiers, etc. en route. You won't need many toys with a young baby; they'll mostly want to look around and sleep. (And you'll mostly want them to sleep.)
* Have a prepared playlist of music that's soothing for the baby but not soporific for the driver. Ella Fitzgerald singing heartbreak songs was perfect for putting Kit to sleep while I stayed alert.
* If no one's in the back seat, consider getting a car seat mirror so you can occasionally glance back and make sure all's well. But they can be major distractions, so use with caution.
* Don't be afraid of side roads and alternate routes, even if they slow you down a bit. It's more soothing for the baby if you drive steadily at 40 MPH than if you sit in stop-and-go traffic on the interstate.
* We got these cling-on window shades and they worked perfectly: easy to put on when it was sunny and take down in the evening, effective at shading the baby. They're great for rental cars and safe in accidents (unlike shades with metal edges).
* Consider a car seat protector to keep the area under and around the car seat clean. It's especially useful if your kid is at the Cheerios-scattering age, or if you're worried about damage to car upholstery. Waterproof car seat liners are vital for kids prone to spit-up, diaper leakage, or toilet training accidents. Get two so if the one in the car seat is soiled mid-trip, you can swap it out.
* If you're staying in a hotel and storing breast milk or pre-made formula in the room's mini-fridge, bring a fridge thermometer to make sure the fridge is at an adequately cold temperature. Remember to pack a little travel bottle of dish detergent and a bottle brush for washing bottles.
* Graco Pack 'n' Play travel cribs are the awesome. They're easy to set up and take down, and they work as playpens and/or changing stations during the day as well as cribs at night. Kit's regular crib at home is a mini crib, so we bring their mattress and sheets along. If that's not an option for you, get a thin mini-crib mattress (takes up barely any space) and some mini-crib sheets and mattress pads that you launder a few times to give them that familiar smell; your baby will sleep much better surrounded by the scents of home.
* Get a travel humidifier for use in hotel rooms, which always have extremely dry air.
* Expect to have two duffel bags worth of stuff for the baby even just for a weekend. Diapers take up a lot of space, especially cloth (disposables have the advantage of not coming home with you). So does bedding, and you'll want extra in case of spit-up. Kit's very drooly right now because of teething and we went through two bibs and four to six burp cloths a day. Bring a laundry bag to make it easy to tell what's clean and what's not.
* Traveling with cloth diapers is a challenge. We used a Planet Wise hanging wet/dry bag for Kit's cloth diapers and it was fantastic--there was no smell leakage at all, even after days on the road, so we could just throw it in the trunk of the car with the suitcases. A smaller wet/dry bag with five diapers went in the diaper bag and was equally useful.
* Don't forget to bring a stroller or carrier for toting the baby around outside the car. A car seat frame may seem tempting, but your kid's already spending a lot of time in the car seat, and it's better for them if you can change it up a bit.
* Pack a first aid kit that includes gas drops, your antihistamine of choice, Tylenol, saline spray/drops and a snot-sucker, and a thermometer. If your kid gets a cold or has an allergic reaction mid-trip, you'll want all your vital supplies on hand. Calamine lotion is great if you're anticipating bug bites.
That's all I can think of off the top of my head, but I am happy to answer questions about anything I missed!