Which is sometimes the only thing to do. Here are two interesting ways:
1. Play basketball. Warning: this ain't easy, so it's a great way to fritter away large chunks. Credit due to the Grand Reptilian.
2. Find out which world leader you most resemble. Idea stolen from Bronson's website. He won't mind as he's a lawyer! Note: you can do this test with four different numbers of questions, and the answers might change, so many minutes can be whiled away. (Though I always came out Gandhi-like, which casts grave doubt on the validity of the test.)