I don't like backpacks in the parks - hate having another layer on my sweaty back! I use a small fanny pack for the parks when I want to bring my camera, and the contents of that pack are already divided into ziploc sandwich bags just to keep them dry - but I've noticed in the past that having everything bagged still makes it easier for the security people at the gates as I just lift out the 2-3 bags and zip through the checkpoint.
As far as a "wallet" is concerned, another thread here on the DIS chronicles my quest for a clear ID holder on a lanyard that will also hold my room key/charge card and a small amount of cash. On days when we travel "light" in the parks, that will be the only thing I take in with me.