I have this camel back that's great for a day pack and I just strap it onto the back of my normal backpack for extra easy to get to storage. I don't take the actual bladder with me but I've found that the pack is perfect for just day hikes and a cooler. This particular camel back has an insulated pocket where the bladder goes and I've used it to keep eggs, sausage, hot dogs, and even cans of soda cold. It works pretty well, I recently got back from a trip up to Big Sur with a friend of mine and we were able to have a nice cold can of Coke two nights in a row. I put one of those frozen ice packs that they ship with medications and things like that to keep them cool, I put that at the bottom, put 4 cans of soda in there, sausage and six eggs were all able to fit in there and stay surprisingly cold. My question is if anybody knows of a good material for keeping things cool because it is sort of a downer having to take my items that need to stay cold on a day hike with me if I don't want to leave my camp spot but want to spend the day hiking around. So my thought is that if I found a material that worked well for this I could just sew a pouch that I could slip into my day pack and pull out and leave at camp. I figured somebody would probably know a material that works really well (perhaps better than another that I would have gotten) for this particular project.