If you work in an office:
15.) Use office supplies to "decorate" your work space. Paper clips make especially excellent sculpture material, for the record, and remember: you have six different color highlighters for a reason.
16.) Desk chair races.
17.) Water coolers are endless sources of amusement; never underestimate the power an upside-down tank of water with hot and cold spouts has to create interesting situations.
18.) Make a desk fort. ("Don't let Accounting in, they have cooties!")
19.) If you work in a cubicle farm, dance down the aisles and keep track of how many people actually notice.
20.) You have high-speed internet, don't you? Well, you have enough imagination to know what that's good for.