My early years on a computer were spent cracking the DRM (I think we just called it copy protection back then) on Commodore video games and sharing them with friends. It was fun and a good learning experience.
What would have likely landed me in juvey today or possibly jail though was when we discovered the Anarchist Cookbook. We had an AP chemistry teacher who was really cool and we used to try to work out some of the formulas for some of the explosives found in there... so we could "improve" them. I distinctly remember one night being out in the middle of a farmer's field with something consisting of cotton balls soaked in something, stuffed inside newspaper and diesel fuel. A curious sheriff showed up and some how believed we were doing a science experiment for class. (Probably because most people building bombs don't pull out sheets of chemical equations to show off.) We also used to buy black powder, ground it, build into various kinds of pipe bombs along with custom fuses.
In college I fell in love with Sun workstations and loved using in the engineering labs. Workstations all in use? Compile a fork bomb and crash SunOS. Need to start your own business? Acquire some university server space for hosting. Need some fake ID's? Get access to the lab with the Tektronix dye sub color printer.