Flat worlds, toroidal planets, cube planets, etc.
My longest-lasting conworld existed on the inner surface of a sphere with a light source at the center. I briefly considered having Yih (the homeworld of the yinrih) be a toroidal planet, but thought it was too out there and decided to give it a ring instead.
i used to have a multi-planar setting that had some weird ones. one i particularly liked was a world on the interior of a cylinder. gravity was parallel to the axis of the cylinder, so all the inhabitants, mustly a subspecies of brightly colored elves, lived hanging off of these cliffs overhanging a bottomless pit. the plane wasn’t infinite, but it was looping, so if you went down or up enough, you’d get back to where you started.