Good / Calm
simmer - To heat a liquid, usually a stew or soup, just below the boiling point, allowing it to cook slowly; or to stay in a slow, controlled state of progress or tension, often before a larger event or change occurs.

He was simmering with anger, but he kept it inside and didn't say out loud what he was thinking and feeling.
tinker - Attempt to repair or improve something in a casual or desultory way, often to no useful effect

Mitch tried to tinker with his computer before realizing that it was more broken that he had thought, and that he had to bring it to the computer store to have a professional repair it.