Others have given awesome advice. I want to urge you to make absolutely sure she is NEVER really hungry. Hunger makes it far harder for anyone to handle anything, and makes it about 1000 times harder for our kids. Have you ever let her have any of the balance or zone bars, the ones that are supposed to be a meal or snack? They are thought of as "health" bars but not all are. We have found that most of these are sweet enough that kids really like them - and if you get the right ones they have enough protein to really help. Protein is a key to helping kids be able to cope. Meals and snacks should be balanced in a ratio of 40% carbs (pref complex), 30% fat and 30% protein to provide the most help. Of coruse the more complex the carb the better. There are two brands of bars you can pretty much rely on to have that ratio - zone bars and balance bars. I have an extreme aversion to most things that taste like "health" food and I really LIKE these. The balance bar gold in caramel nut crunch is the BEST - tastes almost like a snickers. DON"T tell them it tastes like a snickers - it is LIKE one but not exact. They have LOTS of flavors and even gas stations now often carry some of them. My kids actually prefer them to candy bars much of the time. I let them pick anything with that ratio or more protein than that ration. Get some and have them on hand for the trip for a super fast fairly portable "treat" or stealth health snack.
You can get more ideas about keeping food in that ratio from books about the Zone Diet by dr Barry Sears. I don't push it as a diet, but the ideas in there can be really helpful. For example, if you are doing fast food for breakfast, go for an egg mcmuffin - it has more fat than the ratio, but otherwise is really good. If the kids want a candy bar, try giving them 2 oz of lean turkey with it. It will offset the sugar rush and be very helpful. I started this years ago with a group I worked with. The change in how I felt and how I controlled my emotions was incredible after a week or so. I often didn't even need coffee to wake me up! My pms was FAR more manageable and I stopped feeling like I needed to yank someones lungs out through their nose about four times a day. We found that Wiz became far better able to control himself and to cope.
Drinks on the road are usually a huge dose of sugar and not much else. I suggest trying some of the drinks that have less sugar or no sugar. Water, of course, is best but not everyone will drink it all day. Herbal tea can be quite helpful - my kids all like Celestial Seasonings Tension Tamer tea and Tazo's orange something-or-other tea, hot or cold. There are also drink flavoring packets made with stevia instead of artificial sweeteners (which can cause problems in many kids). Stevia is natural and if you use too much it is bitter, so it makes retraining yourself to liking less sweet things pretty easy. The drink packets are under the names True Orange, True Lemonade and True Raspberry Lemonade. They do NOT make a drink like kool aid. The taste is a bit more subtle but still refreshing. They are with the crystal light, etc...
It would probably be helpful to let difficult child pick some of what you will do. Esp if you can get brochures and pics of places and put them into your picture calendar. If she has a favorite toy or blanky, make sure it is in the car wtih her for the entire trip. make sure it is with you before you leave ANYWHERE. It may be helpful to sew a small loop on it that a clip could go on and then use a bit of ribbon and a carabiner or other on-hand clip so that you can attach it to a stroller or your purse or even a backpack if she uses one to carry her stuff. Just make sure the loop is small enough that her hand won't go through it - in an accident this could eman a broken arm or worse. If what she likes about the item is a certain texture of fabric, go and find that feel in the fabric store and sew a small length to her care seat, making sure her hand/arm wont go through. My daughter has a certain silky textured edge on her fave blanky and was a terror in the car if it wasn't with us. My mom made a new cover for her carseat and put a length of similar ribbon on it, anchored at one end. She sewed up the lenthg of the ribbon to make the two pieces (it was a loop) unable to be separated. It was amazing how she stopped fussing in the car.
Most of our kids have significant sensory issues. Does your difficult child seek out or avoid certain textures, sounds, tastes, smells, movement, etc....??? These are some of the things that can help keep her calm. There is a book called The Out of Synch child that explains sensory problems and how to help them. The same author, Kranowitz, also wrote "The Out of Sync Child Has Fun" which is packed iwth sensory activities to help calm various sensory needs. It may be helpful to find ones that your daughter would like (her likes and dislikes can show you what sensory input she needs and what makes things worse just by looking at what she likes and hates to do and why- too loud, too quiet, etc...). Be SURE to have some sensory things for her to do when seh gets upset or just needs them. The Has Fun book can help find those things. One mom here made up a big box full of dried beans or rice (uncooked, of course) and buried small items in there to be found. It can occupy a child for hours if it is the right thing for certain sensory needs. Try getting a small to medium plastic container with a tight lid and putting rice or beans and toys ro whatever in it for her to use when you are not in the car. Sensory activities are amazing when a child is getting upset. It can really help them calm down.
While you are gone, take pics of her at everything you do, everywhere you stop. It will be super helpful next time and can even show her where the stops on the way home are (keep this info in a notebook). Also let her have a cheap camera to use and make taking pics of the family having fun and let her know that you are counting on her to get picks to help protect themselves from those who would scam and scheme to see what they are interested in. Having a "job" to do takes focus on "OMG we are LOST and I won't be able to "fix" it!! and keeps her occupied. there are many inexpensive digital cameras and video recorders to thinking we have him unalbe to surrender. There are even ones designed for kids to use. If there is a Big Lots around, they have a video camera for $20 right now. husband got one for thank you and a new mp3 player for Jess for the same amt today.
Also take books on tape for her to listen to - having them on an mp3 player is great, but be careful that it isn't too loud in her ears. But they can be awesome in calming a child. Check big bookstores for children's audiobooks.
