The "Anyone But You" costars hit up The Backyard Bar in Key West, FL Saturday night ... and what they may have lacked in vocals, they definitely made up for in flair!

Hadley kicked things off with a spirited rendition of Queen's "Bohemian Rhapsody" ... followed by a group performance of Neil Diamond's "Sweet Caroline."

Then Sydney and Hadley brought it home, with a song Glen Powell's "ABY" character Ben would find serene.

That's right! They sang Natasha Bedingfield's "Unwritten" ... which played out in the movie as Ben's secret "serenity song."

It's unclear what the occasion was, but their group was definitely celebrating something as they were all decked out in pirate gear ... and made their way over to an ice cream shop to fill up after their night of onstage antics.