Bottles

Riding a bike is thirsty work and you need to drink plenty of fluids to prevent off dehydration. The most convenient way to carry fluids is to put it in a water bottle and carry that in a cage fixed to the bike frame. You can then drink without having to dismount. It's cheap, simple and you don't have to carry anything on your back. The only downside is that you have to ride with one hand when you're drinking, not ideal for rough terrain!