"Cuphead" is a fun platform game with a very unique art style that only takes up around 5 GB of space.
"Celeste" is a challenging platform game that only takes up about 600 MB of space and has an amazing story and beautiful soundtrack.
"Hades" is a dungeon-crawler roguelike game that takes up around 2 GB of space and has a great gameplay loop and a ton of replay value, plus it's got an amazing art style and incredible voice acting.
"Disco Elysium" is a narrative-driven RPG with no combat and takes up about 2 GB of space. It's widely considered one of the best RPGs ever made, and it's got a great, quirky sense of humor too.