On our 11hr drive with dd last year, who was about the same age, we had a backpack full of toys I had spirited away a week before, some new toys. Also a bag with books to go through (but she was/is really into books, might not work for everyone!), a mini photobook with pixs of all the ppl we were going to see.
I would also recommend a soft tray for the lap, make sure it is crash tested safe if u get one. Great for stopping toys being dropped.
We always saved the dvd for last resort. Always aye lunch/snacks in the car so that stops were only for playtime. Plus it kept dd1 occupied!
Hmm we also made sure to stop & play before dd1 was due to nap, helped her transition to naptime a bit better.
Safe driving & happy holidays!