In my leisure time I like swimming, reading, and watching.
If you are contrasting doing an activity with simply watching then you don’t need to mention the activity:
(= I prefer watching than playing!)
Meanwhile if someone is showing you how to do something, you don’t need to mention the thing that they’re showing you:
However, if the thing you’re watching is something specific, then you need to mention that thing:
This is especially important if the thing you are watching is TV or a movie, because watch collocates very strongly with these nouns.
Indonesians translating ‘nonton‘ need to remember that if you don’t tell your listener what it is that you’re watching, then as far as your listener is concerned, you could by watching almost anything, like watching paint dry or watching grass grow!