I did a heck of a lot of research before I tried drugs at age 19, and then had a lot of fun on drugs for a few years before giving it all up entirely. My main issue with most drugs is that you can get most of the positive effects of the less addictive drugs with training, without the nasty side effects. My favorite was hashish, which I found cleaner and more clear thinking than marijuana, though it's less readily available in America. I did most of my using in Amsterdam.
The exception is probably hallucinogens, which offer some interesting experiences that are probably hard to get elsewhere, and there's some that are largely harmless. I'd advise anyone to be really, really careful about the drugs that are both chemically addictive and tolerance building. Chemically addictive means withdrawal symptoms if you're not using, tolerance building means you need progressively more not to get to the same point. Nicotine, actually, probably gives you the least bang for your buck with a pretty brutal chem addictive/tolerance building mix. Anything that does similar you need to be wary about.
As for the most commonly used things - alcohol and THC (marijuana, hash) - you really, really can get the lower inhibitions, relaxation, creativity, etc. with some mental training. Quitting alcohol was surprisingly easy for me because eventually, I was doing much worse socially on it than without it. When you're comfortable saying and doing what you think sober, alcohol just messes up your reaction times and coordination and makes you fat and sick. The hardest part about quitting was the social aspect, but after about six months when people got the message that I was really done, that fell off too. I always tell everyone drink around me, use, whatever you please, but it's surprising how many of my friends who go out with me eventually quit. It's like a hell of a lot people drink because they're expected to more than because they actually like the effects.
Anybody who wants to use social drugs should take a look at meditation and self-hypnosis, because you can get the same emotional states that you'd get with drugs through a little practice. As lionhearted mentioned, hallucinogens are the one exception.
I'd like to add the following note: for me, at least, while regular marijuana use did not particularly affect my academic abilities, it did affect my ability to meditate, both while under the influence and during the following day.
"Proper" meditation (where you forget yourself and bask in theta waves) is a definite state which one can reliably enter with regular practice. As a long-time meditator before ever trying pot, I was used to having this resource 15 minutes away, whenever I needed it.
I discovered that it was nearly impossible to achieve this state under the influence or the following day. I enjoyed using pot for it's own sake - but, I would urge you to consider your priorities in mental development. I eventually quit pot because the meditative state was so much more essential.
After training myself to remember at least one dream whenever I sleep, a few hours of REM is more than enough hallucinating each day. Some might say it is better to be hallucinating while running about the real world, but I prefer the ability to instantly wake up out of a 'bad trip'.
The other way around. Sleep deprivation and fevers can indeed induce hallucinations, but I would speculate that they are not as pleasant or controllable as hallucinations from drugs.
The exception is probably hallucinogens, which offer some interesting experiences that are probably hard to get elsewhere, and there's some that are largely harmless. I'd advise anyone to be really, really careful about the drugs that are both chemically addictive and tolerance building. Chemically addictive means withdrawal symptoms if you're not using, tolerance building means you need progressively more not to get to the same point. Nicotine, actually, probably gives you the least bang for your buck with a pretty brutal chem addictive/tolerance building mix. Anything that does similar you need to be wary about.
As for the most commonly used things - alcohol and THC (marijuana, hash) - you really, really can get the lower inhibitions, relaxation, creativity, etc. with some mental training. Quitting alcohol was surprisingly easy for me because eventually, I was doing much worse socially on it than without it. When you're comfortable saying and doing what you think sober, alcohol just messes up your reaction times and coordination and makes you fat and sick. The hardest part about quitting was the social aspect, but after about six months when people got the message that I was really done, that fell off too. I always tell everyone drink around me, use, whatever you please, but it's surprising how many of my friends who go out with me eventually quit. It's like a hell of a lot people drink because they're expected to more than because they actually like the effects.