Ponchos, sunscreen, chapstick, water bottles, camera, battery charger, small ziplock bags (for our phones to stay dry on wet rides), granola bars, dum dum pops (to keep kids occupied if needed), small cooling/mister fans, small pack of anti-bacterial wipes, small pill box, Mio (for our water), change of clothes for kids, and maybe an umbrella depending on the weather forecast. That all goes in a backpack. This time I purchased a clear backpack hoping it will make security a bit easier.
I keep our money, credit cards and my phone in a waist bag.