Write a to-do list. Make it as long and detailed and possible. Create categories and sub-categories. Make little boxes next to each item, to be filled with check marks.
Write a long, detailed email to your mom (or dad or sister--whoever). She'll (he'll) appreciate knowing what's going on with you.
Find a passage from your favorite book and copy it, word for word. If you're looking to kill time, do this longhand. (Silly as it sounds, this is actually a valuable writing exercise, as it forces you to slow down and figure out the sentence structure and cadence of writing you admire.)
Create a file detailing all your story/poem/essay submissions, including notes on each one explaining how you felt when you were rejected, if the rejection was just, why you'll never submit there again, etc.
Try to remember the words to an Elton John song (any Elton John song will do) and write them down (this is also an excellent time killer).
Think up a baby name for every letter of the alphabet. Write them down.
Drag a string in front of your cat, back and forth across the keyboard. Eventually something good's got to come of it.
Create a silly list and post it on your personal blog.