Here's a pretty interesting excerpt from a post on Cracked.com:
Hallucinate Like You Just Took LSD, Legally
Yes, that's right kids! Tell your dealer goodbye and worry no more about winding up naked on the roof of an office building after a bad trip. Now you can be stoned out of your mind by building a homemade deprivation chamber out of some regular, completely harmless household objects.
Holy Shit, How Can I Do It!
You are going to need three things: a ping-pong ball, a radio with headphones and a red light.
Step 1: Turn the radio to a station with just white noise (static), and put on your headphones.
Step 2: Cut the ping-pong ball in half and tape each half over your eyes.
Step 3: Turn the red light so it's facing your eyes.
Step 4: Sit there for at least a half an hour.
Step 5: Follow Ben Franklin and your new friend, Harold the unicorn, into the gumdrop forest, and live happily ever after.
How Does It Work?
It's called the Ganzfeld effect, and it works by blocking out most of the signals that go to your brain. It's the same kind of effect you get when looking into a soft light for a while and lose vision, except at a larger scale.
The sound of the white noise and the light from the outside of the ping pong ball are eventually ignored by your brain. With all those signals out of the picture, your brain has to create its own, and this is where the hallucinations come in. We can't guarantee they won't involve, say, the ghost of Lizzie Borden trying to hack off your scrotum with an ax, but that's the risk you take, dammit.