StarTropics — Dip the Letter in Water
Nintendo's Zelda-like adventure with a puzzle hidden in the box.
StarTropics (1990) is the one where Nintendo broke the fourth wall and reached into your physical world. You play Mike Jones, searching tropical islands for your kidnapped uncle, fighting with a yo-yo in a Zelda-style top-down adventure.
Its legend is one puzzle: the game tells you to dip a letter in water for a code. Players were baffled, until they remembered the game shipped with an actual physical letter, and dipping it in water revealed the number. No internet, no FAQ; just you and a wet piece of paper.
A charming, weird, very Western-feeling Nintendo game. That letter trick is one of gaming's greatest packaging gags.