Car seats can be really hot so dress in singlet nappy / light suit, cover with a muslin wrap if need be (can also be used at windows, as a towel and to wrap her)
Have a shade over her window. The sun really bothered our DS so we would block it with a silver sunshade too if need be.
Have room for you to sit in the back if necessary
have a blanket/ towel for her to stretch out on at stops also for nappy changes also useful if you have to change her in the car.
Disposable change mats are invaluable and. Have all nappies etc easily accessible - a nappy cahnge mat like the GR8x one was good - fits in lots of nappies and other bits and pieces.
Pack all drugs, teething chew toys, bonjela etc
If you are breastfeeding keep your fluids up. Remember to move your legs etc to prevent clots
toys attached on a chain/ dummy clip - prevent projectiles and easier to find.
A mirror so you can see her. PErhaps a musical one? I have heard that they don't like it dark at night so my friend used a dvd player. Might be why my son whinged on night trips
. You could try using a small 'red light' as long as it doesn't interfere with the driver/ other travellers.
You can always fly home