A ping pong ball cut in half
Sound of static (fm radio)
Put the ping pong halves over your eyes, the reason for ping pong balls is that they allow a bit of lighht through, put on the head phones and listen to static. After only a few minutes your brain will start 'making sense' of the static and you will start tripping hard, seeing very weird things.
What always gets me is the fact that all the things you see are manifested in your brain, even stuff you have never seen before