The mini-aquadoodle mats are great for little ones. They come with pictures of sesame street, thomas the train, winnie the pooh, and princesses. There might be more, but those are what I have seen. You fill the pen with water so it doesn't make a mess. I keep these with me in the diaper bag in ziploc bags all the time. The kids color the pictures with the water pen and when the pictures dry, they are ready to be colored again.
My kids liked to play with almost empty water bottles at that age. We discovered that out of desperation when we ran out of things to occupy my DS one year. DH handed him a bottle of water that he had been drinking and DS loved it. He shook it, banged it, squished it, probably chewed it, lol! This was the same trip that we handed him the
AAA map to play with from each state that we went through on the way home. He shredded them into a million pieces but it kept him entertained for quite a while.
I have also brought a large tupperware bowl full of little people stuff or match box cars. Pick a bowl that is the right size to fit in their lap in the carseat. This way they can play with them in the bowl and not so much goes on the floor (unless they dump the bowl, lol). A cookie sheet with magnets usually entertains them for quite a while. Just be sure that you choose large magnets if your kids still eat things. We used disney character magnets from previous trips last time but you could probably use any magnets you have on your fridge or ABC magnets.
We did a lot of singing kids songs and disney songs on the ride and I usually bring a beach ball for them to play with at rest stops (blows up quickly and takes up little space).
Will they do stickers without eating them? Maybe you could find a sheet or two of the large stickers with characters on them? Even if you only gave them one large sticker they would probably like pulling it on and off their shirt or pants. My 18 month old loves to peel stickers off the sheets and stick them on a piece of paper, or her clothes.
HTH!