Shape:
* Cube: A pyramid has a triangular base, while a cube has square sides.
* Sphere: A pyramid is pointed, while a sphere is round.
* Cone: A pyramid has a flat base and triangular sides, while a cone has a circular base and curved sides.
Structure:
* Hut: A pyramid is a large, monumental structure, while a hut is a small, simple dwelling.
* Building: While both can be structures, a pyramid is a specific type of building with a distinct shape.
Other:
* Flat: A pyramid is a 3D structure, while something flat is 2D.
The best antonym will depend on the specific context in which you are using the word "pyramid."