what's this driveway around lawn designed for?

No idea, but it looks nice. You could put a tree there or a sales table. 

Or you could always grab one of the model cars from B/B, size it up a bit and then place it so it looks as though it belongs to the family. I'm guessing that's the Summerdream fairy family's lot, though, yes?

Round about bike lane? Like the ones for cars but this is for bikes.

Oh, I know ... It's a small staging area for Gnomes? 😁

Makes me wonder if this isn't an Easter Egg, which is the term for a hint for the future. . . Just sayin'. Doesn't make it so.

Duelling arena?

Oh, yes. I like that answer, mariefoxprice83​ 😊⚔️😁

i think that's outside of lot, how to custom things in world? i don't use cc/mod.

You can place down in world the objects that go inside your sim's inventory, things like bike, tent, fireplace, sales table, etc. Might need to try a few times until they place it in the right spot though.