I've played plenty of FPS games. Though, I haven't played many simulator games. So, I thought it would be a cool idea to playtest a game that combines these two genres.

That game is Zombie Graveyard Simulator. Which's gameplay is tied to whether it is day or night. In the day it is simulator game where the player needs to do jobs in a graveyard. At night, it is a wave-based shooter akin to Call of Duty Zombies. I really enjoyed this dynamic though the game had some pretty gnarly bugs.