Wake from sleep using the power button only?

Rather than doing a full shutdown when I go out for a couple of hours or to run errands, I'd really just rather put this thing to sleep. But I have a cat, so the few times I tried that I'd come home and the machine would be awake because at some point he had jumped onto the desk and hit either the keyboard or mouse.

I tried putting the machine to sleep and then pulling the USB cable for the keyboard/mouse, but that causes it to wake for some reason.

So, is there a way to put the MacPro to sleep and then have it ignore all other wake attempts other than to hit the power button again?