I recommend bringing sunscreen, a bottle of water, a light jacket, and comfortable shoes with good support. The terrain is uneven, and doubly so if you're climbing the pyramids. I wore a pair of comfy, stretchy sneakers, but found myself wishing I'd opted for hiking boots when I almost twisted an ankle climbing the Pyramid of the Sun. You will want water, but if you forget to bring a bottle with you, water and snacks can be purchased from vending machines near the museum. There's also street food, as well as the famous La Gruta restaurant not far outside the gates. Just remember to hold on to your ticket to re-enter the site! The street food options are good for those on a budget, so it's really not necessary to pack a lunch unless you have dietary restrictions.