For just 32kB (plus the size of a Game Boy emulator), you can play the amazing Tetris Rosy Retrospection. It's a romhack of the Game Boy Tetris that adds modern Tetris controls, handling, and features to make it feel much better to play without increasing the file size. I'm aware of the color version of this hack, but it doubles the file size to 64kB, so I'm only considering the regular Tetris Rosy Retrospection this time. Byte-for-byte, I can't find a better game to sink dozens of hours into.