The game mode at it's core is survival, but there are plenty of Easter eggs that act like puzzle quests that you solve on the map, all the meanwhile trying to survive at the same time. The reward for doing these things come in the form of achivements/trophies, new weapons, perks, etc.
yeah ive been meaning to play with slay too, i normally dont play too late on weeknights either considering im up at 5am, weekend nights anytime though
poor slay has to wait till end of feb or even march for new dlc, that blows