The best advice for this that anyone can give I think is that if you're not on Mousebits, get on there now! It really is the easiest way to get all of this music.
My absolute #1 recommendation for music is the
Epcot entrance loop! It is essentially an overture for the park and there is so much great music there... Soarin', Golden Dream, old Living Seas music, UoE, One Little Spark, etc.
Background Music:
Epcot entrance loop
Innoventions Plaza loop
MK Main Entrance loop
Main St loop
Tomorrowland loop
Frontierland loop
Jungle Cruise queue radio loop
Muppetvision queue loop
Hollywood Blvd loop
Attraction music:
Splash Mountain
Carousel of Progress
Pirates
Mansion
Muppetvision (plus the awesome pre-show!)
Illuminations
Soarin'
Finding Nemo: The Musical
Festival of the Lion King
RnRC

The Trolley Show from Main St - I don't think anything better sums up the MK so succinctly
A road trip is also an excellent time to discover a new podcast. My current favorites are:
WDWFanboys - dumb, but harmless
Tom Corless's show that I'm not supposed to link to - good recurring segments: Match Game, Back to the Future, Good Bad and the Ugly, and Top Tens
WDWRadio - Lou Mongello's; very pixie dusty but still good
Magical Definition - if you don't mind Jim Hill