I may have missed this in the other responses, but finger puppets are great. Last year our granddaughter (now 3) would put on silly shows with finger puppets on her fingers and toes. It was great. Also since you will also be traveling with a 5 yo, she can help with eye spy games. We usually do colors or letters or items that she can see over and over since the little ones sight is partially blocked from the car seat and she may not be able to see the original item as quickly.
Another silly game we play is to ask her which direction we are going and how fast. For instance when we go over an overpass was ask if we are going up or down, are we going fast or slow (she loves the Little Einsteins so it is usually Allegro or Adagio).
Another thing I never leave home without is her potty chair! It rides in the back and if she says she needs to go, we don't have to look for a place! Liquid can just be tossed on the side of the road to evaporate and solids we wrap up real tight and dispose of at the next stop! Be sure you have wipes instead of tissue since you won't have water to wash up with! She would rather go there than in a public restroom which is just fine with her grandpa and me.
I also make a list of Bible and nursery songs that she likes and so I don't forget them all. Grandpa even gets to participate in the sing-a-long. There are some great DVD and CDs to sing with.
When we stop to eat, we usually do her favorite EIEIO's (McDonald's) and let her play then eat in the car. Just something else to keep her occupied! And we travel on her time. It usually makes the trip about an hour longer but worth not having the tears!
We will be making the 7 hour trip to Galveston in two weeks! And I will be taking a hand help tape recorder. She gets into telling a story and I don't want to forget what she says. Plus, her mom and dad won't be joining us on this trip, so they will be able to join in on the memories and it really helps when journaling in the scrapbook!
Hope you have fun on your trip!