Side note - I'm not a huge fan of plastic bottled water in a hot (or cold) car, but these little packs from WalMart have worked well for us. Noticed them maybe a year ago and we've used them for a while now. They're small enough to where they don't take up a lot of room. The smaller individual sizes serve a dual purpose - we keep some for "just-in-case" but also have some on hand for the kids if they get thirsty. Plus since they're wrapped up in a nice little package, they don't roll around and make a mess.

Hard to tell from the picture, but they're only 10oz, so smaller than the average bottle, but bigger than those single-gulp things you find at costco.