Wow, I'm sort of jealous. I've never hallucinated unintentionally before, though I've recreationally done almost all of the medications cited in this thread. I always had to pay for it. Throughout college I had a rather strong affinity for hallucinegenics, so yeah, I've had my share. Most I don't remember vividly enough to describe, but, in addition to an auditory hallucination that I can best describe as a "sproi-oi-oi-oi-oing", like a spring snapping back into place, but slowly, that I always got, there were maybe four that I can recall.
The first was a summer night following freshman year. A bunch of us at some mushrooms at my friend's parents' house. Big, beautiful house that backed up to a clear-water marsh on the South River, clear night with glassy water. I think I spent the whole night on the upper-floor deck, just watching the full moon slowly sink into the Bay, and the stars dancing around, first individually, but later, the constellations themselves seemed to get their acts together to play out their little stories. Fantastic.
The second was, I guess, Halloween, sophomore year. I dosed a really irresponsible amount of liquid. At one point, I decided to have a shower, and became fixated on the water running down the drain. That translated into the whole world being sucked into some sort of black hole or vortex for a following undetermined period of time, which eventually started to really freak me out. A friend then took me out to one of the quads. It was a beautiful, clear, crisp Ohio night and the full moon was bouncing off the light frost on the grass. At which point the blades all came alive, like little Medusa snakes. I didn't know what to do! I didn't want to step on the little snakes, but I sure didn't want to be there in the middle in case they turned on me! Quite the dilemma! It did turn a bad trip into one of the best nights I'd had up to that point though.
Another was at Virginia Beach, just after Halloween. We drove down to drop, and just wandered around. The thing about VA Beach is that there are (were?) these great, vividly colored murals all over everything, and, because Halloween had just passed, most of the windows and buildings were also decorated for that. Tons of eye candy! I spent a lot of time with this mural, which I swear to god, looked just like the swimming fish screen saver:
Of course, I was also unprepared for the heavy military aircraft presence, and being steeped in X-Files lore, the first black-ops type helicopter that came along freaked me out a bit.
The last time I did something like this was a couple years after college. We walked down to a sculpture garden and I guess I fixated on one particular sculpture of some freaky goddamned rabbit that looks like something out of Lewis Carroll's nightmare. The damn thing followed me around all night after that, though we did eventually come to an understanding.
At this point, I'm almost certainly done with hallucinating for fun. I'm probably far too damaged by now to have a good time with it anymore. It's a young person's passtime.