I don't know your situation but I would suggest if you are feeling down try getting in to some physical activity. For me rock climbing is where it is at and if you are a stoner and fairly in shape you will love it. Don't ask me why but nearly every climber I know does or used to smoke weed, they seem to go hand in hand. If you get into it you will get loads of cool friends and have something that will become an important thing in your life. It is also seems to stop people (me and mates at least) going over board on the chemical pleasures. :rasta:
If the above sounds like a shit idea how about having a year off college and going travelling. If you want to go travelling it gets harder and harder as you get older.