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.

https://www.youtube.com/watch?v=exhe6lsfooM

Then comes Humankind Open Dev Beta, available between October 21st and October 28th.

https://www.youtube.com/watch?v=HQTwKCwYjh8

Today on the 22nd will followed Immortals Fenyx Rising, available until October 29th.

https://www.youtube.com/watch?v=6I6QQibftw8

source: https://9to5google.com/2020/10/21/stadia-demos-immortals-fenyx-rising-pac-man/

For me if it isn't free it isn't a demo =p
Unfortunately I don't have interest in Stadia so these aren't the games that would entice me to test it.

Meh, none are killer apps. Google needs to take this more seriously. But I’m glad they aren’t. 

Stadia is irrelevant, the only ones who seem to have missed that are Google themselves. With new consoles arriving and MS with others diving even more in the PC market again, things will only get worse from here on out.

Well, but it *IS* completely free. Just open the links in Chrome and you can play them.

I know people don't like the idea of Stadia and don't want to sub to it or buy games, but if there is a free demo of a game on there it is completely free as you said. All you need is to open Chrome and have a gmail account to play. I'll be playing the demo, and if I like it I'll pick it up on Switch.

You do not need a game console or a decent PC to play this, just a good internet connection with Chrome browser and a gmail account.

This is actually the one thing I can give credit to Stadia for, is when they have a demo it is COMPLETELY free. Think about it, if a demo is exclusive to PlayStation but you don't own one, then you're either left out or you need to spend upwards of $300 to play it. Now with that said I still think demos shouldn't be exclusive, I'm just saying that if a demo is on Stadia then it is completely free.

I'm playing Yakuza, would take a very major great game to make me think about stopping it to play even if a free demo.

Sure. But for anyone who considers Fenyx this is an opportunity to try it out already, even if they buy it for another platform.

I've actually been considering using Stadia recently to play the latest Baldur's Gate as my computer can't even run it (I just have a laptop for work so it's not too surprising) but I'd like to give the game a try with some friends. Only issue is it doesn't seem like that game has a free trial and I'm unsure if multiplayer will work between those playing on windows and those playing on stadia.

Yep, that is true. If I was interested in the game certainly I would try it over Stadia and also know how Stadia would work for me. So far I didn't see a need to test it.