In high school I downloaded a game boy advance emulator and would play old Pokemon games during class. There's even fan made games you can download called "Rom Hacks".

I think there are Nintendo DS emulators for phones nowadays too. I highly recommend the Professor Layton trilogy. It's a bunch of different types of puzzles that'll keep you occupied for hours. And there's a fun story attached. It's on the app store too, but rather pricey https://apps.apple.com/us/app-bundle/layton-trilogy-bundle/id1522008740