My kids were little a long time ago, and we never used electronics to keep them occupied.
We used recorded music (Sesame Street was a fave), often borrowed from the library for variety. Talking books were popular, especially the ones with the books to 'read along' with.
We found a used copy of a game called Are We There Yet (cards with things to find along the road)... but we played that anyhow --finding motorcycles, counting motorhomes, looking for the alphabet on signs and license plates, etc.
I drove a 5yo and a 2.5yo to Colorado by myself once and the #1 fave toy I bought from the dollar store before we left was a cheap makeup set. Other things we picked up included tiny pencil crayons and notepads, a lapdesk each, 'magnetic' vinyl stickers with a scene to place them in, and the ever-favourite toy of all time: about 1/2 the playmobil in our house. We took 2 rubbermaid bins of it, and it was used in the car, the hotels and at my sister's house when we got there.
We stopped a lot and played with skipping ropes and balls in rest stops, to work out the antsy pants, and they slept a lot more during the day than they normally would.