Broadway musicals are a lot of fun, and you can sing along. You can get soundtracks or "kiddie chorus" versions. How about some Gilbert and Sullivan? Also consider folk music, and kiddie folk like Trout Fishing in America (they also record for adults). Wheels on a Big Rig and 6 were huge faves in our car (as well as "all I want is a proper cup of coffee made in a proper copper coffee pot")