On our drive down last summer, we located McDonald's with play areas approximately where we planned to stop for lunch. We would unload the kids and make the necessary bathroom stop. Then, DH would pick up food for the two of us and we would eat while watching the kids burn off some steam in the play area. When we were ready to go, I'd wash up the kids and get them loaded into the car while DH would buy their lunches. They ate while we resumed our trip and then they were ready for 'nap time' (all parents favorite time of the day).
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McDonald's website is very easy to use and provides directions so that you can be sure you're choosing a location that's right off of the highway (and not 15 minutes out of your way). It even lists locations with drive up windows (if the kids are sleeping when you are hungry).