I carry water with me on a daily basis, so that's what gets used in the car (not stored in the car, but carried in fresh each day).

For food, I don't worry about that too much in my daily suburban life. But during winter I've been known to throw in an unopened box of saltine crackers and an unopened jar of peanut butter. Obviously that's emergency rations only, but they might get me by in a pinch. For "trunk food" (read: emergency only), anything that requires hydrating or heating is overkill for me. The object is not having to gnaw my arm off in hunger, as opposed to holding a first class banquet. I just can't see myself whipping out my stove and cooking up a can of beans in the library parking lot.

I would do more if I lived in a rural area or was heading off into remote areas where I could be stranded for a couple of days. But that's not typical for me any more.