DS love books with flaps, and one of those will entertain him for a while. Also, we have a Vtech "laptop" that is our go-to car toy (usually for shorter trip).
DD is only a year but she loves books that have buttons to play songs. She'll entertain herself for at least 45 minutes in the car with one and then usually falls asleep.
I am a seriously paranoid person, but I try not to give heavy/hard toys in case of an accident. I always think of the stories about how badly a kleenex box in the back window can injure a person in an accident. Crazy, I fully admit. (I am also the person who thinks about having the diaper bag stuffed under the seat so it isn't a projectile in an accident).
We usually use the Fisher Price dog that sings in the car, and some of the mini books. I also try my best to time the start of our trip with morning nap .. and about the time he wakes we do lunch, run around a bit. Then he plays with his toys for a bit and frequently falls asleep a second time. However, we haven't driven more than 6 hours in one trip so far.
Car seats for the Littles would be very happy with you. Not sure if you are familiar with the facebook group, but they do not recommend having any hard objects ie toys, sippy cups, books etc in the car just for that reason.
Re: 18-Month Car Trip Toys