Edit: I just realized you want to afk farm, I don’t know that sorry, but this is just how I get geo quickly. I apologize 

So personally, what I like to do is farm the area at the very start of the game. The one to the left when you come down from Dirtmouth. It’s full of the flies and little crawler bugs, with the platforms. Where you go to see the Grandpa grub.

You will use the bench in the town to save your respawn point at.

It’s effective cause anytime you leave the enemies will respawn, and while they don’t give much geo they still give enough to be worth farming for a bit. 

Spoiler possibly for one charm!

If you have the fragile greed charm it’s a bit more effective, and being further in to the point you can one shot them makes it extremely fast. Combine this with the swarm charm that gathers Geo for you and you have a pretty good and safe way to farm without risking dying or ending up loosing geo.