In OSX, when the screensaver is activated, I don't want to be able to deactivate it with a mouse, just with a keypress. But I don't want to enter a password. I just want to be able to deactivate the screensaver with any keypress but mouse movements should not have an effect. Can that be setup?
The only quick and easy way to do this would be to disconnect the mouse.
According to this its impossible for the computer to differentiate between USB mice and USB keyboard button clicks, when asleep.
if you are somewhat crafty and technical, and If you don't mind using a hardware/analog approach... here is my quick suggestion..
if you dont mind taking apart a $10 usb hub and soldering a rocker switch, then before you leave you can flip the switch, (which unplugs the usb hub) and let the computer fall asleep on its own...
waking it back up would just mean flipping the switch, you computer would recognize that your usb hub and thus your mouse and keyboard were plugged in and wake up