a day at the zoo

It's been extremely warm here in Michigan. We've had 65 degree days for the past few day so my little lady, her grandmas and I plan on going to the zoo on Wednesday. I haven't gone out with her long enough to worry about feeding her so I don't know how to heat up her food. I give her both formula and breastmilk so does anyone have any tips or tricks for warming up bottles of either while out at the zoo?