Haha, y'all are much more appropriately prepared than myself!
Let's see... Well, my horse is currently in Texas, so it's much cleaner than it usually is! The common denominators in my car are fast food cups (I get Hi-C after long nights of partying... and those are typical), my gym bag (which includes a towel, bathing suit, shampoo, conditioner, running shoes, shorts, tanks, hair ties, etcetera), and of course my serving apron/book.
In the trunk I still keep a grooming kit, my horse's lightweight blanket, and empty bottles of his meds. I miss him so much ;-;