We're pretty new to this too...but here's our list of "must haves" so far:
Our Camelbacks. It holds 1000ml of water and functions just like a back pack. So by bringing it, I always have enough fluids and can carry all my stuff. Even if it turns out to be a mostly-in-the-car day.
I have a baggy full of "swag" I guess, stuff for the kids, smashed pennies, foreign coins.
I have a baggy with several spare batteries
I have a few extra baggies. I also carry a few plastic bags, for picking up trash, or if nothing else, to throw muddy shoes in when back at the car.
I have a small notebook, two pens.
I carry bug spray (necessary!).
I have a flashlight.
Any geocoins or travelbugs that I'm moving that day.
I also have stuff in my pack that may someday come in handy, little sheets that dissolve into soap, pack towel, lighter.
I totally need to get some gloves...some stuff out there....ICK. My husband this weekend found a cache in a hidey hole, and it had been taking over by an ant nest. Heebie Jeebies! Glad it was him. I don't mind getting dirty, but getting crawled on is different! I like the idea of mirror on a stick thingy too.
Anyway, this is what we start with, I am guessing we'll be adding more as we live and learn.