Cybercrime and gaming are the most known Internet harmful activities. Cypercrime is in violation of the criminal law and one of the first cases of the Internet harmful activities somehow "taken care" by the justice system. Gaming is probably the most spread and still taken at the level of a joke despite the growing evidence that it is worse addiction that cocaine practically imposed on our children (how many parents would encourage and facilitate the use of cocaine for their children?)
The other activities are:
- identity theft (a part of cybercrine),
- support for terrorism (who regard themselves freedom fighters) hence cybercrime
- darknet (gray area - probably not regarded as a criminal activity)
- science destruction by fake conferences and predatory journals (no responsibility of any kind),
- pornography,\- child pornography (criminal activities)
- "viruses" (criminal law).
I wonder if you can add to it something you have encountered or even heard about...