I sat in the back seat for a while then switched with hubby. It gives the kids a break from one or the other parent. We had a narrow long box that fit between the seats where her feet would normally be, and we kept favorite toys, books, etc...in hands reach. Singing songs is fun too so take some tapes or CD's, which ever your vehicle uses int he stereo. We borrowed books on tape/cd from our local library too.
Stopping every couple of hours is necessary. Even if it's just to run around in an elementary school yard. Babies have so much energy to be used up. Crying and having a temper tantrum is one way to deplete the energy build up, running around is another, I choose the running around...it's MUCH, MUCH less stressful.
Stopping at McDonalds, or a place with a playground inside appropriate for toddlers, to let her have some playtime is extremely important too, let her have a parfait for snack time or some apple dippers, it doesn't have to be chicken nuggets every time. We always figured we'd get one very healthy meal a day with peace and quiet, that was pleasant. We selected breakfast because it sets the tone for the day, gives your body the needed nutrients for the day, and the other meals will be more stressful due to people being tired and bored. Have an ice chest in the vehicle too, it can hold milk and juice and water so she can drink what she needs and not get dehydrated. Also fruit is a good thing to keep in the ice chest too, just on top of the ice with a had towel over the ice to keep it dry. One thing I always worry about is choke hazards. Please be cautious about the size of bites. Riding in a car is different. The motion of the car is there, we are used to it and little ones aren't. Using a knife in the car might be a no-no so think about using a plastic knife, a food chopper, or something else like an apple slicer with a very small cutting board to chop the fruits and/or fresh veggies. The smaller her bites the better off she'll be.
You will be happier to if you get a portable DVD player and put in a dvd of her favorite show, I bought some Nic-Picks from Wal-Mart and it has a whole slew of their top cartoons, one right after the other, just one top episode. Of Jimmy Neutron, Rugrats, SpongeBob, Danny Phantom, etc...they have all kinds. They are a few minutes long so the attention span is easy and they can be turned off after a few and restarted where you left off. You can also check and see if they have any from your local Educational channel, ours is called OETA, it's public TV. Sesame Street, Barney, Word Girl, etc...are shows that might be on DVD somewhere.