traveling West on I 80 to Yellowstone with a 4 year old.

Roadside America is a great suggestion - we found a lot of things to just see driving by. The statue of Mary at the Nebraska Wyoming border was impressive.
Also, checkout "letterboxing." Best website is Atlasquest. Will keep them hoppping and there's always one near where you are. I print the clues out before we leave on trips and we hit the ones that are convenient or when we need a break. Message me if any questions :)