Gaming

When you ask for something without 'grind' I have to ask if you know what you are asking. Grind is entirely subjective. It's not a mechanism of a game but rather what happens when you personally don't find a game mechanism fun/rewarding.
Take classic examples, like mining in... most games, really. It's smacking a rock. It doesn't have much variety. For some people, they love their own little game of 'hit the rocks in the most efficient way,' or they like to relax with music and bust rocks, or they feel like every rock is a loot box. Other people hate it for being too complex to automate and too simple to feel engaged.

The difference between 'grind' and an 'endlessly replayable part of the game' is how the player looks at it. You are asking for 'the drug to which you will never build a tolerance.'