Here's a few fun things you can do with your senses
none of them illegal, yet
(but to be safe, check your local listings).
Your visual system has two basic aspects:
foveal vision, related to the tinier-than-you'd-think sharp area at the center of your eye, aka the fovea centralis; and field vision, or the entirety of the visual field.
Supposedly most color sensitivity is just in that tiny central area, but it's not as light sensitive as the field system: if you very carefully stare exactly at a star, without wobbling, the star will disappear.
Most people rely almost exclusively on focussing on the center point: where to put it next, what do I want, where is it. However, if you let your awareness go out to the whole field, strange things can start to happen.
Mainly our visual system senses change; if you focus on a point onscreen and leave your hands on the keyboard, at first you can still see all of your fingers, no problem. After a bit, there's just a kind of vague blurriness where your hands were; try wiggling a finger, but don't move your eyes. What do you see? It's not a finger, it's just a weird kind of squishy bugginess down at the edges. Now, quickly pass your eyes down to see your hand and then back up to the screen: your hands reappear. While your eyes are still, observe what happens around the edges of your visual field, too.
There's something called steady stimulus de-sensitization (SSDS). It's one of the principles behind isolation tanks: take away external change and you are stuck with whatever's left. If you ever get into a truly dark room, like with no light at all (like a cave), open your eyes wide and move them around; it shouldn't take long before you start seeing structures, even movement. One of my favorite things to do is lie under a tree canopy at night, and pick a single leaf to fix your eyes on. Don't stare at it necessarily, just use it as a reference to keep your eyes from moving. Let your attention go out to your entire visual field. If you can go without blinking for a bit, after maybe 10-15 seconds your visual system will get bored/fatigued, and a kind of phase-shift will take place, and bang, it looks just like aciiiiidd. I've managed to get up to about five consecutive levels of shift before; if there's a slight breeze it can get downright sublime, extremely psychedelic. Move your eyes just slightly, and the leaves and sky reappear immediately. Works great with stars, too: just pick one and leave your eyes on it, but open your awareness to the whole field. Once when I did this the whole sky just swooped down on me and did a little dance, nice.
One thing you'll also notice is that the less you try to really grasp what's happening, the size of your visual field will actually expand. If you're trying to capture and remember it, you get a smaller cone, but if you just let it go, it gets much wider.
Which brings us to the next exercise, which takes a little more doing: you simply ignore your visual system's input, just completely cease to care about it in anyway. Let your eyes cross and go completely out of focus. The more completely you can give it up, the less inclined you will be to move them, at all. It's actually kind of hard to do, but when you get it, an especially odd thing happens: first everything kind of fades to black, then you somehow re-emerge into a much stranger field. You can even expand this to include as many of your senses as you can get to, just lie comfortably, loosen your clothing, and somehow, ignore your senses, Even ignore the fact that you are "trying" to do this, and especially continue to ignore any fun changes that happen. If you slip off it, just go back to it: don't care, so what, piss off.
Here's a neat trick to trigger lucid dreaming, where you know you're dreaming, and manage to stay aware of it. Depending on the level you get to, you can change the whole scene to whatever you want. During your regular waking life, anytime you find yourself waiting at a stoplight or otherwise killing five seconds, or even just whenever you think of it, add three numbers together mentally, then do it again. Try to get in the habit of doing it a couple of times a day even, especially when you're absolutely sure you're awake. The idea is that it will become a background habit, then some night when you're just walking along some Honolulu street unaware of the continuity error, fully confident that you're wide awake, you'll do your little simple-test thing, and it will fail miserably, the results won't match, the numbers won't even stay constant, and you go oh shit! At this point, don't freak, and try hard not to get too sucked into any detail, because then the lucidity vanishes. One thing you'll notice is the amazing depth of field, everything is so sharp...I once looked up and saw billions and billions of stars in the sky.
Ujjayi breathing is an excellent technique from yoga, really a great thing to keep available at all times; whenever you feel a twinge of anxiety, just go to it. Hell, anytime you think of it, just go to it:
to learn, first, open your mouth, and as you exhale, close your throat slightly to make a slight glottal hissing "hhhaaahhhh" sound, i.e. not with your tongue tip, but somewhere in the back, near where your tongue meets your palette, in your throat. Now, close your mouth, and try to do it. Once you get this, try to also do it on the inhale. It's a little awkward at first, but quickly it becomes quite effortless. Ultimately what you want is an even level inhale, and an equal even, level exhale, with an audible (to you anyway) gentle hiss. In yoga classes where this is emphasized and everyone's breathing is synchronized, it can be pretty incredible. Once you get this, you can give yourself a decent little buzz in about five focussed breaths.
Another angle on it, from my Yoga section:
Even simpler, and even more powerful, is this. Yes, it sounds really cheesy, but done well it’s quite the handful. On the inhale, imagine that the energy is coming from absolutely everywhere, and as it comes in, think to yourself "YES! YES! YES!", letting the energy fill you and mingle with all of you, becoming your life. As you exhale, release it all to everywhere, die just a little, and think to yourself "thank you, thank you, thank you." By about the third breath you should feel your entire body contracting and expanding. I tend to get so blown out by it that it's kind of hard to focus enough to do five complete breaths, it's that effective.
for much more on this subject, try the excellent "Stoned Free: How to get high without drugs", by Patrick Wells with Douglas Rushkoff