I'd definitely say going with Steam is your best bet. It has a 2 hour refund policy for any game you purchase, and they're pretty quick with it. Not sure how much you should rely on that when perusing, but it's an option if you end up picking a game that doesn't suit your tastes.
As for games... I honestly don't know. There's so many to choose from and different genres that I couldn't really suggest anything concrete. Don't even really know what your background with gaming is... But here's a couple games.
Super Meat Boy is a really solid 2D platformer, if you're interested in that kind of thing. And if not, it gives you a pretty good idea of what you're getting into off the bat, meaning you can refund it in under two hours.
Could always play Portal. Another game that shows you what it's made of pretty well from the get-go. Then if you end up liking it, you've even got a second game set up to play! Maybe even a third, but that's a fan-game that got propped up by the devs, I believe...
People have said a lot of good things about Hollow Knight, but I've never really gotten too far into it. Also not really a Steam exclusive, so... Eh. Never hurts to try, right?
Same goes for Baulder's Gate 3, but that's a preeeetty long investment, from what I know. Also kind of a niche type of game... I think. If you like DnD you could possibly enjoy playing it. No idea how intensive it is to run either, but if it turns out to be too much for your computer's specs, you can at least refund it.
A Hat in Time is a cute 3D Platformer. A bit kiddy, maybe, but it's fun enough. If not, you'll probably figure it out quick enough to refund it.
Nier Automata is good, but it's a bit dour. Also might take a while to get into, so refunding could be a little tricky if you find it isn't for you. Also not technically Steam exclusive, and not very well optimized for PC, but it's still a pretty good game.
I'd say Garry's Mod, but... Boy are there a lot of quirks to that game, even if there's just as many ways to play it. 'Quirks' like having to buy other games just so objects in the game don't come up as giant ERROR signs and purple+black patterned tiles.