I second the small thermos of hot water. I used to take a thermos wherever we went, shopping, zoo, museums, etc. When my kids decided they were hungry, they didn't want to wait for me to find a microwave or stand in a line to get hot water. I just filled the thermos with hot water and brought it along with some bottles with powdered formula already premeasured in them. I poured water into the bottle (bring a small funnel along), shook the bottle and instant warm bottle!! When you think about the amount of stuff you already carry around when you have a baby, a few extra items is not a big deal, especially if you have a stroller, which I am assuming you will.