Or take "watching cartoons". You might discover stop-motion and start making your own. Or if you got into Anime you might get interested in learning Japanese.
The Sudbury schools emphasize the power of boredom. When you don't play videogames that much you might FEEL like you could play them all the time if allowed to. But if you were ACTUALLY allowed to, you'd eventually get bored and want to go deeper. And then you'd inevitably be learning stuff. A repeated discovery was that most kids eventually DO decide they want to learn math, in part because it's something other people can do that seems like it might be useful. And when kids are actually interested in the subject, they can learn it in a really TINY amount of contact hours and it doesn't harm them at all to have waited a few extra years.
The reason standard schools waste so much time teaching math is that they're trying to force kids who aren't interested at all in learning it on the school's schedule rather than their own.
As for "english grammar", you can learn that just by using some of your freed-up time to read lots of books.
Here's a story that involves teaching 6 years of math in 20 hours of class time: