I bought a small backpack/purse at Target. (I think it's an Eddie Bauer one) It has two drink holders on the outside (ESSENTIAL!), and several little pockets for various things. I always put my wallet in the biggest pocket WAY at the bottom and when I zip that pocket, I make sure both zippers are way down to one side so that it would be difficult to unzip from behind.
It fits in all of the ride pouches and, like someone else said, you don't have to take it off to go to the bathroom! It's also big enough to hold some cheap ponchos for the wet rides and a couple of small souvenirs and snack items. I love it!