I'm going to try very hard to describe this. It will not be enough :)

It's more than "fun", much more profound, it's Everything, perhaps the single most important thing. It's ineffable, yet, you still feel obligated to try and explain it, as I'm doing. You may experience total happiness, love, wholeness, but then seconds later experience the exact opposite, total depression, despair, loneliness, humiliation, fear, violence. You may experience a plane of existence where you ARE time, and you see how time affects everything at once, as a result, it's a feeling of timelessness, you can see everything happening, a feeling of eternal existence. Perhaps you will then see that the same way events are connected together to form the feeling of time, in fact, everything you know is connected together in your brain, you can move through any of the connections, not just temporal. You may experience realities similar to your own, perhaps many at once, but you may also be catapulted into a completely geometric and abstract universe of inexplicable nature, but you'll feel as if you understand it, as long as you don't try and describe it in language. You may "see" the fundamental structure of sound and music as your various neural compartments cross-fire and visual cortex processes sound or sound cortex process visual data. You may "see" sexuality. You may see good and evil and the necessary duality of the two. For that matter you may see the dualistic relationship of everything. Everything exists because defining non-existence commands the definition of existence. You may see the fundamental feedback loop where perception meets the internal mental model your brain builds to interpret reality. Wait? feedback loop? You don't even know which side is real, cause-effect is completely broken down, it's now a Strange Loop. You may then see, truly see, that your whole entire known reality, including everything you see and perceive to be "outside" of your body is all actually inside your brain. You're trapped, this is your reality. You will likely discover/meet your inner self, you will realize that the consciousness really is the tip of the iceberg as you get rocketed inward into your deep sub consciousness, which has a mind of it's own. Also, for me, it knew everything I was thinking in advance. So when I'd make a joke or expression it would react with "why are you responding to me, I'm the one that told you what you think", so you learn to just shut up and enjoy the ride. It will likely be so beautiful and significant that you may even question if what you experienced was a divine experience. It will be so intense that you will have had a glimpse as to what a super consciousness or hyper-intelligence may look like, as you become even more aware of how small your conscious mind really is in comparison to the whole. You may feel as if all, everything, every person is connected. For example, the knowledge of good vs evil, and the gradient between, and hot and cold, and temperatures between, are these really different things? After all, in your brain they are both represented as neural connections, a graph if you will. And is it just the interpretation of the neural pattern that gives them meaning? The brain just being a massive network of brain cells, where the fundamental unit is a connection, you may realize, no wonder the foundation of human reasoning is also limited to the notion of stateful logic.
Perhaps, after 10 hours of this, you start to want to come back to reality, but you learned something very important this day. You can't go back, you don't, part of you has transcended to Buddha-hood. But you're a stubborn human, you fight to come back, that fighting, rooted in fear, fear is linked to many other negative memories, they may take command of your brain. Have you went crazy? Did you just forgo your conscious existence as you knew it? Then you're sitting there, watching yourself in a mental institution as your family is there crying wondering what happened to you, but communication with them is futile as you have transcended, but part of you, is stuck in that body, for 50 years as you've realized that this infinite experience is just a bit more than you asked for. But that's just one of the many negative things you dream up! Of course, acid is illegal where you lived, so that random sound is the cops showing up and escorting you out. Or perhaps you wake up naked and vulnerable outside, but it's just a trip, as you rock back and forth between the meta-mind-space and all these false realities. (a process called "looping") You face all your fears and inner personal problems, you confront your selfish ego, and all the negative things about yourself, you're embarrassed of yourself as your mind is nakedly exposed. Just as you got to see all the beauty of nature, you also realize how messed up you and everything is. It's hard, very hard, but you face it and accept it. This is what some people call a "bad trip", but to me, that is precisely what made it a "good trip". You eventually grab on to pieces of reality, one at a time, until you finally come all the way back. It takes many hours, and days to process everything. You're never really sure if you ever came back. But you don't mind. You've experienced enlightenment. You're at peace with yourself and the universe, if there's even a difference. You have a new appreciation for everything. But similarly, you also realize the empty, void of meaning nature of the universe. For some people this is all too much. For me, this was the ultimate lens into "reality", and gave me a new spark and energy for life that is unparalleled to any previous experience.

A small side note: This feels very great to have finally shared. It has been melting inside me to get out.