Open up retroarch and apply the following as settings for a game:
adjustment filter to mirror the screen, I think it’s in an image adjustment folder but can’t check which one at the moment
swap left and right in the controls (in-game remap, not the menu controls)
Mirror mode! On any game! As long as you don’t care about text, it’s a fun way to add replay value. Great for platformers like Donkey Kong Country 2, Mario, etc.
If you really want a mindfuck, play a top down game like Zelda Link to the Past with the above but ALSO top down inverted too. I do that with the ALTTP randomizer sometimes.
Edit: hang on, I got Yoshi’s Story at launch and I 100% remember the ultimate aim of the game is to actually get all the melons. It’s not an alternative mode really, it’s the actual goal for 100%. At least, it’s how I played it in 1998.