I'm a native Dutch (Flemish) speaker. For me 'Niksen' means you're doing something without intent/purpose and with low importance. You can watch a movie, just because you don't know what to do and you're filling the time, maybe your mind is wandering to other things while watching. Or you can watch a movie because you want to see it, like going to the movies instead of watching at home. Only the former is 'Niksen'.

'Puttering' is an English term which I feel is closely related to 'Niksen' but not exactly the same. With 'Puttering' there's always an action. But with 'Niksen' you can be sitting in a chair, looking around, 'doing nothing'.

I would even argue that meditation is not 'Niksen', because with meditation there's intent. It's actively calming yourself down.