1.
Mischief is playing harmless tricks on people or doing things you are not supposed to do. It can also refer to the desire to do this.
The little lad was a real handful. He was always up to ~...
His eyes were full of ~.
N-UNCOUNT
2.
Mischief is behaviour that is intended to cause trouble for people. It can also refer to the trouble that is caused.
The more sinister explanation is that he is about to make ~ in the Middle East again...
N-UNCOUNT