Puyo Puyo (Game Boy)
This game has a hidden level select.
A slow-paced, laggy port of the arcade version of Puyo Puyo.
At the main menu, choose the first option, then at the screen that appears, hold Select and press Up a number of times equivalent to the number of the desired level (e.g. 2 times for level 2). Press Start to go to the chosen level.