We did a 20+ hour drive halfway across the country at least once or twice a year while we were in the Marines, with two kids.
The biggest hit with the boys (anywhere from 1 to 4 years of age) was MAPS. We'd pick up the freebies every other state or so (whenever we stopped tried to make it at the tourist info ones) and they LOVED those...my then 3.5 year old "read" me the map for 2 hours straight on one leg of the trip! That time was extreme, but usually the maps were good for 20+ mins of entertainment. They "read" them, looked for different color lines on them, drew on them, shredded them (in the case of the 1.5 year old), etc. Just make sure you have multiples, because if your kids are like mine, they'll want new ones when they accidentally rip one
Also popular were coloring "books"...I printed online coloring pages of their favorite characters, or whatever they were interested in, punched holes in them and put them in a 3 ring binder (gave them something sturdy to color on). I also included blank pages for coloring/scribbling. I usually had plenty of crayons and colored pencils on hand too, and they each got their own brand-new box of crayons before we headed out.
For the young, young ones I did random small toys, as well as plenty of snacks and drinks (plenty of snacks and drinks for the older ones too).
Also, we travel in the most comfy clothes possible...usually light sweatpants and tshirt, and they travel with whatever they take to bed with them every night. Even when they were past naps, they would typically fall asleep for at least an hour after lunch.
When we drove over two days, we always stopped at a late-ish (7 pm or so) supper time so they had plenty of time to run off energy. Other than that, and an hour long break for lunch (at state park/rest area type thing with outdoor playground, again to run off energy) stops were very short, just long enough to get gas and everyone to potty. If we did the drive in one day we usually still did a late-ish supper around 7, but stopped for an hour then too, and put everyone in pajamas before we got back on the road.