For a day hike, I have a fanny pack with 2 compartments, between the compartments is a pocket for a bicycle water bottle. The night before I fill the bottle 1/2 full and freeze it. This type of water bottle is thick enough that it will not bow out when frozen. In the morning I fill the rest with water and I have ice cold water for at least 4 hours.

This bottle is translucent, not the clear polycarbonate bottles that leach BPA.