The FacelessTech blog posts about making custom cases for Gameboy cartridges and including an OLED display inside:

I’ve been making custom 3d printed cases for Gameboy games and I thought I would combine some electrons in this time. I wanted to add a screen to the Gameboy game but It would just display the game title and be completely stand alone from the game.

If you remember the ghost cart which took advantage of the small size of the cheap repro flash carts available online. That got me thinking, There is a lot of unused space so I did some measuring and through together a PCB that combines the Arduino pro micro and a I2C connected OLED screen. The whole stack up is a bit thick but it would still go into any Gameboy because the top part of most Gameboy games are exposed at the top so you can pull them out.

See the video below and the blog post.