Awkward situations are not what you want on your vacation so figuring out who should pay for what should be done before you leave.
The best way to figure out who should pay for what is to not even figure that out! The best way is to decide how much you are going to need for your vacation. Once you have that figured out divide that by the amount of people in your group and that is how much money each person needs to put in the pot. This should make it easy by everyone adding there share to an envelope or whatever. Have one person control the money and they can be the one that pays for meals and such that are group activities. This will make your life much easier and stress free!
Maybe there will be money left over that you will get back!