Like many people in my generation(and the previous two!) I have had some very positive and interesting experiences with psychedelic plants including Cubensis mushrooms.

  Despite the controversy about these drugs,  a quick internet or library search of legitimate medical resources will disclose the fact that these substances are not addictive, do NOT cause organic brain damage and are physically non-toxic.

The effect on the brain's neurons is to increase the frequency & range of signals traveling between brain-cells. The subjective experience is of dramatically increased detail in all senses, which can become disorienting if it exceeds one's ability to keep up with it all.

History and archaeology shows us that for the Greeks, Mayans, Native Americans, Siberian nomads, Aztecs and others the Psychedelic plants(mushrooms, morning glory, san pedro, lysergic amides) were considered as a sacrament for gaining spiritual insight and set apart from other drugs which dull the mind or addict the body.

Research indicates most "hallucinations" have some organic or biological or sensory basis. You don't just see pink elephants on parade, you have incredibly detailed and complex omni-sensory experiences which are often difficult to describe or interpret.

 To the Experiencer these perceptions are of such a high level of Order and Beauty that they virtually DEMAND a better explanation than simply being the random result of scrambled brain chemistry.

Over the last 17 years I've been blessed with the opportunity and temperament to carefully experiment with the following psychedelic compounds:
Stropharia Cubensis (psilocybin) mushrooms,
Ayahuasca (DMT family of Tryptamines),
Yopo seeds (Bufotenin & various Tryptamines)
Salvia Divinorum (salvinorin A),
San Pedro (mescaline) cactus,
MDMA (Methylenedioxymethamphetamine)
5-MEO-DMT,
LSA (Hawiian Baby Woodrose seeds)
and LSD,
I have compiled a list of recurrent trip experiences which i have personally experienced in enormous detail, that happen frequently enough in various people to suggest they may be subtle perceptions of real non-hallucinatory events. Some of these are listed as common perceptions in the clinical research books, LSD Psychotherapy by Stanislav Groff PHD and Varieties of the Psychedelic Experience by Dr. Robert Masters & Jean Houston.
 
Examples of Super-Subtle Non-hallucinatory Perceptions

Visual grid patterns or very fine Pixelization
This may be a perception of the distribution of the rods and cones in the retina OR maybe the million or so fibers of the optic nerve which carries visual messages to the brain. *UPDATE: found this on the web - "One author tries to account for the different form constants by referring to the various structures in the eye. He concludes from anatomical and observed data that,"the rods and foveal cones can look backwards and that the retinal pigment and the choriocapillary circulation can, therefore, be seen under certain conditions" (Kluver, 65). In essence, some of our 'hallucinations' are views of looking backward at the retina."

Flashes in the sky at night
This may be due to the extreme dilation of the pupils, especially outdoors at night one might be more sensitive to faint atmospheric "heat lightning" as well as flashes from the thousands of small meteors which come in every hour. This is testable by recording the sky with night vision gear and seeing if the flashes are caught on video.

Feeling the cells in your body, nerves in your teeth and the inner surfaces of the sinuses.
The central nervous system stimulation experienced on psychedelics can be extreme, sometimes to the point of experiencing awareness of every nerve and cell in the body, DNA awareness is a common theme today and coiled snakes(!) were a common theme in ancient cultures. Can it be a coincidence that Nobel prize winning geneticist Kary Mullis credits his discovery of how to artificially replicate DNA(polymerase chain-reaction) to insights visualized during an LSD trip.

Hexagonal patterns in certain cloud conditions.
On a trip years ago I saw amazing intersecting hexagonal patterns in cumulus clouds. Later,  in a book on Chaos theory I found a diagram of hexagonal convection currents(when hot air rises in the middle and the cold air around it sinks) in the atmosphere caused by the sun's warmth. This was exactly the pattern I had seen on a hot, cloudy Mississippi day

Seeing Bones, Veins and muscle through the skin.
I have read reports and directly experienced this. The acclaimed artist Alex gray has done a series of amazing paintings inspired by this experience. Seems to happen most often in florescent lighting, possibly because of the way they highlight blemishes, acne and veins. Human skin is not opaque; hold your hand up to a bright light and you can see the bones. Can psychedelics temporarily increase the brain's ability to perceive slightly higher frequencies of light? This would be easy to test.

Brain processes
In Physiology class we learn that the brain has specialized areas which analyze visual information in terms of color, form, motion and depth. Some have noticed during the peak of a trip seeing a rapid sequence just color,  just form,  etc. Sometimes associated with perceiving the "frame rate" or "sampling rate" of the visual sense, which is around 80 frames per second(?).

Seeing clearly the refresh rate on a computer monitor.
This is definitely not a hallucination, but is an artifact of the time dilation experienced on psychedelics.
When one second seems like 2 minutes there's plenty of time to notice the 60cycle refresh rate on a monitor, the frame rate of a film projector or the strobing of a fluorescent light.

Access to childhood and dream memories.
Clinical LSD research in the 1950's-60's indicated that one of the psychological effects was the
activation of suppressed or emotionally charged memories from childhood.

Coherent Wave patterns in rivers, lakes, etc.
Here's my theory on this. It is known that psychedelics inhibit serotonin and this causes neurons to fire more often and carry signals to more distant adjacent neurons. One result of this is increased pattern comprehension. When viewing a large body of water in this state one sees all waves traveling in a given direction simultaneously, in fact the mind will automatically attempt to track the direction and momentum of ALL the visible waves simultaneously. What you see is a freakishly orderly march of interpenetrating wave crests which seem to vibrate. Sometimes it looks like all motion stops and the water is a solid, vibrating rippled surface.

ESP, Shared thoughts and synchronized experiences.
The reports thought transference are prevalent in modern and ancient psychedelic literature, In fact one indole alkaloid was actually given the chemical name Telepathine. Also shared experiences of internal imagery are not uncommon.

Timeless Moments.
I have noticed a very subtle phenomena in regard to event cycles. It's that brief moment when EVERYTHING pauses. The conversation pauses as the wind pauses as the fridge fan stops, as the CD is over, as the cat sighs. A mass synchronicity of relaxation and pause.

Increased sensitivity to body language/ visual cues.
Sometimes tripping can cause  extreme awareness of subtle posture/gesture/face muscle and eye cues. Entire conversations can occur just through small gestures and facial expressions.