NiksenvsMindfulness

The short answer

Mindfulness is directed attention to the present, with a technique to practise. Niksen has none — no method, no posture, no goal. One correction to the story usually told here: the contemporary Dutch dictionary does gloss niksen as 'bewust niets uitvoeren', deliberately doing nothing, so the intentionality is not purely an English import. The 1956 dictionary gloss was the plain 'niets doen, luieren', and even the deliberate version leaves nothing to do.