I did a similar thing. I bought a Gameboy and modded it for music production, but by far the most time I spend with it was playing Tetris DX where I started from failing really early up to the highest levels (that give you the rocket launch ending). It is a terrific device to play Tetris. No colour, no distraction, 30h of pure Tetris with one set of batteries.

I am however missing that you can't start from higher levels than 9 (thus forcing you to replay boring bits) and it is missing the modern Tetris features like holding and hard-drops. Somewhat surprising nobody implemented modern Tetris for the classic GB yet.