These days, the smallest music selection I travel with is on my iPod Shuffle, currently at 99 songs, 7.3 hours, 526.6Mb. Here is a sample of what's on it.
It's the kind of song you need to zone out while crossing the flatlands on a coast-to-coast drive. The tune is mellow so I can stay comfortably under 80.
Read the lyrics and you'll understand why this is a fit for some journeys.
On longer rides I often tune into the XM Chill channel. I like Thievery, De-Phazz, Bitter:Sweet and many others.