Pranks
My office is pretty fond of pranks. It helps that my boss is a jokster. Here are some that happened in the last couple of years:

For someone's birthday, just before Christmas.

Old backup tapes that are to be destroyed, but not before baricading someone's office.

For some reason, one of our colleagues had pine cones on all the tables at her wedding. So of course, when she came back from honeymoon, this is what she saw:

What my boss finds in his office when he comes back from a week away hunting:

And of course, the masterpiece, the tin foil wrapped cubicle: