Hijinks are examples of mischief or merrymaking that are often disruptive or rowdy. Prank phone calls and gag gifts are both forms of hijinks.
Hijinks is a fun-sounding word for activities that are fun: getting into trouble, but not serious trouble. Throwing a football around homeroom could be considered hijinks. Burning down the school would not be considered hijinks; that would be considered a felony. If the pesky neighbor kid rings your door bell and runs, you can say, "Oh, he's up to his usual hijinks!"