So here's what we do for rest stops. We drop two ropes a decent ways apart from one another and have some races. Whoever wins gets a delayed start for the subesquent race, etc. So all ages can participate. We do this for about 8-10 min and are back on the road, without any major detour to get off the highway. The next stop a few hours later, we take a mini-football out and toss it around for about 10 minutes. And maybe have a short cartwheel contest. Again, we don't have to get off the highway.
Contrary to Wadecool's experience, we've found enough room separate from the dog walking areas to not have a problem. And, most folks seem to keep their pets in the dog walk area vs. all over.
Now, when my ds was younger (he's 5 now), we would plan one off-highway break per day, and we found that outlet malls were pretty good distractions, and usually had a playground as a bonus. The promise of new sneakers on a trip used to delight him.

Don't ask me, I'm just his Mother!

Maybe someone will know if you'll be heading by an outlet center along your route.