Bubble Bobble (Game Gear)
This game has a hidden level select.
The level sizes are extremely cut down due to screen limitations, but hey, at least you can still tell it's a game about Bubblin' and Bobblin. Also extra stuff.
Fun piece of trivia: this port was developed by the South Korean company Open Corp., known for releasing almost exclusively unlicensed games, which itself was formed by ex-employees from Zemina, another South Korean unlicensed games producer. What makes this a particularly odd situation is that Zemina had produced quite a few unlicensed ports of Bubble Bobble, one of which was also released on the Game Gear! Maybe Taito was impressed by their work?
Enter 9S5CLNN3 as a password, then select 1P Continue in the main menu.