We did an 8 hour drive when Ellie was 4 months old for a wedding, but there were 6 adults in the van with her so we could take turns entertaining her, plus she was so little, she didn't do much other than eat and sleep.

Now, I'd probably try packing new toys that she hadn't seen before as well as her favorites, and play music that I know she likes as well has have snacks available for her.