Our culture seems to love treasure hunts. We hunt for Easter eggs, search for the pot of gold at the end of the rainbow, and of course, watch Johnny Depp collect pirate booty. Maybe that's why Amy's annual anniversary treasure hunt seems like such an endearing idea—at least until it isn't.
The treasure hunt was originally a tradition for Amy's parents, which probably means it was doomed from the start. After all, there are no two marriages more different from each other that we know of than Amy's parents' and Amy and Nick's (for more on this, check out the "Characters" section).
Amy's dad always hid the clues for her mom, a fact that does not go unnoticed by Nick: "Don't think I don't see the gender roles here," he tells us, "that I don't get the hint" (3.26). In other words, it's an act of passive aggressiveness from the get go—Amy telling Nick that she has to do the romantic things in their marriage that the man is supposed to be responsible for because he's not capable of it. And since the clues are never easy for Nick to figure out, they serve as an additional way for Amy to let Nick know he's not the man she wants him to be.
So instead of the treasure hunt representing what Amy and Nick's life together is, it almost represents what their life together is not. It isn't a life in which they value the same moments—otherwise Nick would figure out Amy's clues, no problem—and it isn't a life in which they are in perfect—and traditionally gendered—harmony like Amy's parents. Instead of an annual celebration of their marriage, then, it's an annual reminder of Nick's ineptitude as Amy's husband.
The treasure hunt for their fifth anniversary, therefore, is like everything else related to Amy: a duplicitous mind trip with a double meaning. Like Amy's diary, like her friendship with Noelle, like Amy herself, it's two-sided. One side is sweet and charming, while the other is insidious and vengeful. Amy is a wolf in sheep's clothing to the worst degree, so it makes perfect sense that she'd hide her plot to frame her husband for murder in the super sweet and ultra romantic packaging of an anniversary treasure hunt.