Long drive

If leaving overnight isn't an option(we're driving in a group with a couple different cars and leaving early morning), what easy car activities did you plan for your 4 and almost 2 year old? My younger daughter will be 20 months when we make our 20 hour road trip. I'm not too worried about our 4 year old, she's fine with anything we give her but I'll happily take activities for her too(: