I always use my "water wallet" at WDW. I just hang it around my neck and have plenty of room for our safe key, KTTW cards, IDs, and a little cash. DH is so used to having his wallet that he takes a cheap velcro one and just has a little cash in it (I have his ID in the water wallet). He just says he "feels funny" without his wallet, but the one he carries doesn't have anything important in it.
My uncle had his pocket "picked" at WDW years ago. They caught the kid. Yes, kid. The family was working together. They had the kid bump up against people in a crowd thinking people wouldn't think anything of a little kid (he was only 6 yrs. old...how sad). Anyway, Uncle felt the kid get his wallet, he turned around and yelled at the kid, who took off. WDW security caught him and subsequently, the family. They found 5 more wallets on these people that didn't belong to them.
I guess that experience made me wary of carrying anything "valuable" in the parks.