Clash Royale on mobile probably qualifies if you spend no money. The levelling/card collecting grind will take months or years to hit max level, but the matches are quick and interesting and you can jump in for a few at any time.
Playing the game seems to be the object of it, not the grind. There is little reward for constant play (about 8 wins a day is optimum, with each match taking 3-4mins).
It's quite fun as a casual game, with some deeper strategies and builds available, and not bad even at lvl 1-3 to get some good matches in (I'm level 5 now, at 3rd or 4th arena, with an ok'ish set of cards/units/spells after 3-4 days of too much play. 70-80 wins out of about 140-160 matches or so? A 50/50 win ratio still allows reasonable "progression" of sorts).
A weird mix of both quick and longterm play, with playing good matches being the reward. Sponge off guildmates to open up your card set a bit and you'll end up with enough variety and strategies available within a few days to keep it as fun enough to log in a couple of times a day for ages. Quick match-making, quick games, "reasonably" balanced, and so p2w to even think of getting to the higher brackets that you never have to care about being in them (more cards really doesn't mean better player). Good for a morning coffee/quick lunch break/arvo wind down game.
Mini-micro-dota, with a way easier headspace to play in. It's free as well. Like super-p2w/advance upwards mobile game free. But so far I haven't cared too much (good play can often beat higher levelled people). The p2winners float to the surface naturally, leaving you happily fighting against free scrubs like me
Forever!!!!!
(until one of us breaks down and spends some money on it. Which I probably will at some point. Until then, there's something slightly zen about the lessons of patience and the virtue of being a ninja goblin (or something) there for us all to learn. And it's rewarding learning
)