Who could forget the Muppets?
Try "Movin' Right Along" from the first Muppet movie.
I personally find that getting your favorite songs from your CD collection is the best bet, because you'll get the songs you love and aren't afraid to sing along to. As far as classical goes, get the bombastic kind because the subtleties are lost on a car radio.
I also find that trying to "theme" your tapes provides different styles for different moods. For instance, I made several mix tapes for a journey from Spokane, Washington to Eugene, Oregon, and tried to match the tapes to the scenery. (I didn't have time to go beyond four, but those four actually worked pretty well.) The first tape was light and bouncy and appropriate to leaving someplace and starting a journey. Then the next tape (and the next) were more appropriate to driving through the Palouse, which is pretty much farmed desert.
The key is providing enough variety to avoid boredom and highway hypnosis. Do this by occasionally throwing a "wake up!" song in there, like O Fortuna from Carmina Burana or For Science by They Might Be Giants.