Left 4 Dead 2

Left 4 Dead 2 is a first-person co-op zombie shooter from Valve. Four survivors fight through five campaigns across the cities, swamps and cemeteries of the American Deep South, from Savannah to New Orleans. Each campaign feels like its own horror movie, ending in an evacuation.

The standout feature is the AI Director, a system that arranges enemies, weapons and medkits differently every time. Besides ordinary zombies there are special infected that strike without warning, so the team has to stick together.

The campaigns take about 10 hours, and a 100% run about 63 hours (according to HowLongToBeat), so a month at 2 hours a day easily covers the campaigns. To be honest, the real strength of the game is online co-op and versus; the rental gives offline access only, so you play the campaigns alone with bots and multiplayer is not included.