If you want to spend a bit more money you can also go for something like the Ultimate Challenge Practice Lock. I have one and it's been a pile of fun/frustration. And as Squelchtone said, the Master No. 3s are also fun. Just make sure you don't pick the one your wife relies on. The look she gave melted one of my picks!
"We all sit around in a circle and suppose, while the secret sits in the center and knows." --Robert Frost