Is there anything better than finding a couple dollars sitting in your coat pocket that you forgot about? Well, after checking my trophy list for BioShock: Infinite, I realized that unlocking 1999 mode is actually an achievement. Hooray me.
The achievement trophy is called "Should Auld Acquaintance", and instead of beating the game in easy, normal, or hard difficulty, you can just use the Konami code (found here) to skip right to 1999 mode and get that trophy.
Click here for more achievements/trophies for BioShock: Infinite.