Well here are two fairly easy hidden achievements for Bioshock 2.

 

9-Irony - Attainable in Ryan's Amusement Park. Simply use Telekinesis to throw a nearby golf club at the model of Andrew Ryan sitting at his desk in the beginning of Ryan's Amusement Park.

Savior - Save every little sister without harvesting any of them and spare Grace, Stanley, and Gil.

 

And there you have it, say hello to more achievements.