Have you seen these Platypus water bottles yet? I just bought one for my brother's birthday ( on August 1st, so don't tell him!) and I think I may have to get one for myself!
They weigh just about nothing, take up very little room ( I mean, roll it up and put it in your pocket small) and they are BPA free. When filled with water, the bottom of the bag becomes rounder, and fits perfectly in a cup holder... This is one of those things I never knew I needed until it was created. What will humans think of next!