The beautiful thing about mixtapes -- or, more recently, mix CDs and custom playlists -- is that they can be whatever you want them to be. A little bit country, a little bit rock 'n' roll? Go for it. A track you could fall asleep to up next to a song that makes you want to party? Sure, why not! Something from the '60s followed by something from the '90s followed by a current No. 1 hit? Totally cool.