Check if the door is locked a few times.
Gift the object to gain entrance to the train engine.
Get ending A.
Get ending B.
Get ending C.
Get ending D.
Get ending E.
Get all three joke endings.
Find 15 coupons.
Find all the artifacts.
Tag all your friends, then dispose of the bat.
Speak to Joe in the aftermath.
Collect every achievement for Summer In Memoria.