What about a random "enter", random multiple "arrows up" (when writing text, that would be nice), or a Tab when filling out forms.

The ALT-key used in applications activates the menu. An additional user key might activate a special function :->

A random Num-Lock is quite good for laptop keyboards only, because the number keys are shared with the normal alpha-keys. However USB devices on laptops are too easy to detect, unless the user uses a USB-hub.

Laptops on USB-hubs might like extensive use of the brightness up and down function keys.