When I was in high school, I heard this thing about getting "high" through blood pressure and oxygen manipulation. The technique (don't try at home):

1. Be seated.

2. Actively hyperventilate for 30s. Deeeeep breaths.

3. Stand up quickly.

4. Immediately stick thumb in mouth with full lungs and attempt to blow air out as hard as possible without actually letting any out.

5. That's it. That's the whole thing.

So, feeling dubious, I tried it while hanging out with some friends. I stand up, do the thing, and in <5s boom. Dreamland. I had what felt like a full 6-hour experience, semi-lucid dreaming. I had some control and awareness that I was dreaming, flew around some giant yellow and pink mushrooms. I dont remember any other details at this point, but it was a whole day pretty much.

Wake up to my head hitting my buddy's guitar. I somehow had an hours-long experience in the time it took for me to pass out and hit the ground. Haven't done it since, but man, I don't think I'll ever forget that. One hell of a ride.