So, to recap, you want a substance that makes boring things enjoyable, keeps you alert and able to function, does not cause addiction or tolerance, is cheap and, let me guess, leaves your health mostly intact.
Don't kid yourself into thinking a substance will do all that for you without taking anything away. When it's not the difficulty of the experience, it's the toll it takes on some aspect of you. I think your best bet is exercise and good food, for real. Healthy living can cause addiction, though, so be warned.