Stadia offers free demos for multiple games for a limited time. These games are available for the basic Stadia which is free, so you can just fire up your chrome browser and play these games.
First up is a demo of Pac-Man Mega Tunnel Battle, which is available between October 20th and October 27th.
Then comes Humankind Open Dev Beta, available between October 21st and October 28th.
Today on the 22nd will followed Immortals Fenyx Rising, available until October 29th.