Start a battle
If you have some free time, you can use it to start (and try to win) a war against your colleagues. Take some toy guns and get your revenge for all the disappeared lunches or loud phone talks that distract you all the time.
And if there are no plastic guns in your office, you can follow the example of this New York office and use something that you most certainly have on your desk—colorful sticky notes. These guys have created a giant portrait of Marge Simpson and one really cute (and big) Pink Panther.