I can't NOT play the Binding of Isaac: Rebirth.
Pretty simple in concept, you start as Isaac, your mom is a religious zealot and wants to sacrifice you, this makes you really sad so you run into your basement crying. You start out with 3 hearts, 1 bomb, you move with WASD and Up/Down/Left/Right shoots your tears in the appropriate direction.
6 Floors at the start, although the true game really starts once you beat those 6 floors once, then you get 8. I believe the total number of stages is 10, but I'm not sure as I haven't reached those final 2 levels yet (need special circumstances)
Its basically a dungeon crawler, each time you start a new game, the levels are randomly generated, and the enemies in the rooms are randomly generated. Also, when you die, you die. You need to get through stages 1-8 in one life, unless you get an item that gives you another (few and far between)
You start out just shooting basic tears, but each floor has a golden room with an item, again randomly generated. Also, at the end of each stage after you beat the boss, an item appears. You could get something that makes your tears shoot faster, bigger tears, laser tears, fire tears, poison tears, anti gravity. You could get an item that gives you a heart container, a soul heart, a black heart, etc. There are I believe 150+ items in the game, and they all do different things. Some are even actives such as spawn a companion or instantly clear a room.
So each game is pretty different. Since your items stack, I could have an Isaac with tons and tons of health who shoots really fast fire tears, or a 1 health isaac with extra big bombs, etc, etc. Combinations seemingly endless, and always random.
There are something like 8 unlockable characters by doing assorted things, such as clearing two levels without getting hit, collecting 7+ heart containers, etc. Adds to the replay value, as these characters start differently than Isaac (one can fly for instance so you can pass over barriers, one gets +1 damage for every -1 health you take, one has an extra life, etc.)
You get stuff for beating the game with everyone. You get stuff for beating the game multiple times. If you beat the first 6 levels in under 20 minutes, you unlock an extra "Boss Rush" stage that needs completed for achievements. There is a hard mode, you get stuff for beating it on that too. More items you unlock, more variance in gameplay. There is something like 178 achievements to get in the game.
Best part, its only $15. You can easily pick it up, play a run or two in under a half hour, and just put it away. Watching TV, can play during commercials. Its pretty low key, pretty fun, and TONS of replay value from all the randomness. You aren't required to finish your run in one sitting too, so if you have to leave it will save your progress at the last room you entered.
I've got something like 58 hours on this game, and only have 54 of the achievements unlocked....lot of game to be had, can't recommend it enough.