I would go for the simple spitting in her drink.

Or, I have used the stinky hand method before. "Mallrats" was the inspiration for this. Discreetly, stick your hand down the back of your pants and wipe you ass with it, then shake their hand, touch them, or an item that they often touch.

This one i haven't tried, but i have seen it done Bird seed or bread put on their car at the beginning of the shift.
Disgusting satisfaction!