Which Nancy Drew books are great for road trips?

Depends somewhat on your itinerary and destination, and your attitude.

For example, if you're going to a vacation at a lake, "Secret at Mystic Lake" might be exciting...but it's about crime at the lake, so that foreboding might not be so good for your stay there.

If you're traveling across the Great Plains, "Strangers on a Train" might upgrade potential tedium.

Any of them because they are essentially the same book rewritten with a new plot.

Id' recommend Nancy Drew #176: "The Mystery of the Weird Smell Coming From the Backseat."

drastically outdated. I read them in the late 1950s-early sixties