C.B.
Hi M.,
I would get a lot of little toys. Keep them with you and whenever she gets bored or irritated show her a new one. If you trade with her then you can just rotate them. Finger puppets are great, because then you or your husband can perform a play as a distraction when she really gets frustrated.
I kept a bag of toys by the kids car seats so that they could pick and choose. Also include lots of board books with colorful pictures. Change the toys often because then they look forward to seeing what they have to play with.
Also, I have found talking in the car really soothes. If you are passing lots of trees point them out, or big trucks. When my son was about your daughter's age we went on a six hour trip with my dad (who does not like stops). My son was "big Truck" addicted. Everytime one passed by he would poing and say "big twuck, big twuck". There were so many he finally wore himself out and went to sleep.
I hope my suggestions help you. Have a great trip. Take lots of stops, if it gets to be too much.