Recreational explosives.
Mixed ammonium nitrate and fuel oil. Then set off a nice strong concussion wave next to it... Managed to launch my heaviest model rocket to date, but it only went up 20 feet or so. It had the rough mass of an tree with a 2.5' diameter trunk. For some reason a number of cops came and inspected the remains of my model rocket and it's launch site. There is now a McMansion on the launch site, but at the time the kids were able to fill in the hole and play soccer in the field.
My other hobbies included, but were not limited to, sewing, designing clothing, photography, astronomy astro photography, learning, painting, programming, digital electronics, computer logic design, trading stocks, reading, bugging professors trying to find out what to learn to do stuff, and listening to adults who were talking about stuff like banking and finance, what they were doing at the local research reactor, and stock trading methods. Basically anything a very precocious kid could do to pass the time and not think about how bad he was being treated by nearly everybody from town.
What did explaining computers do in cave man times?
Inventing the wheel, of course
Nah, knapped stone tools, slings, spears, blow tubes & darts, bows and arrows, etc.