Warning: the following techniques have not been tested on actual productivity.
Extreme Pomodoro
Work for 25 minutes without a break, then take 5 minutes to stare into the abyss.
Example: 25 minutes of intense focus, followed by 5 minutes of existential dread.
Daydreaming
Let your mind wander, but only after you've set the timer.
Example: 10 minutes of intense focus, followed by 10 minutes of wondering if your cat is plotting against you.
Video Games
For when you're really, really stuck.
Example: 30 minutes of progress, followed by 2 hours of 'just one more level'.