1. The cube is made of a material that is very hard, somewhat reflective, and almost black -- somewhere between smoky quartz and obsidian, with a surface rather like one of those stones when polished. It has some variegation of color, and the faces are very slightly concave. It does not have a size as such, since there is nothing to give it scale. It hovers off the "ground" by a distance equal to about a quarter of its edge length, and rotates slowly clockwise. It is viewed from slightly above and to the left; I can barely see the top of the cube. (By the way, the background of my mindscape is black, as the night sky without stars and moon.)
2. The ladder is metal, with a dull polish -- perhaps stainless steel, or even titanium. All its pieces are cylindrical. When it appears, the cube stops rotating, and the ladder rests against the left face of the cube. Its top rung is just above the top of the cube, such that one could use it to climb up onto the cube.
3. The horse appears on the right side of the cube. Assuming a horse of standard dimensions, now the scene has scale. The cube height is about 50% taller than the horse. The horse stands about 8-9 feet to the right of the cube, and is facing toward the back left corner of the scene (I see his left side, at an angle). He is a relatively ordinary looking chestnut color horse, well groomed and healthy, no saddle or tack. His manner is very laid back. He simply stands there, gazing with casual interest at the cube, occasionally nibbling at what must be some grasses that appeared with him.
4. The flowers are interesting. They force some light into the scene, not much -- perhaps full moonlight. The light reveals that the entire scene has a background of mostly small wildflowers of diverse type and color. They are mainly behind and to the sides of the cube, not in the foreground, and the area in which the cube, ladder, and horse stand is free of flowers, like a small clearing.
5. When the storm comes, it turns darker again. There is thunder, lightning, rain, wind. Not hurricane-strength, but a decent rainstorm. After it passes, the ladder has fallen on the ground alongside the cube. The horse had walked up right next to the cube to seek some shelter, and is now taking a few steps away and shaking the rain off himself. Many of the flowers are bent over, some in standing water, with water on their blooms and leaves. They are hardy little things, though, and there is a sense that the rain will help them grow. The cube itself is unchanged except for being wet. Free of the ladder leaning against it, it begins to rotate again. The horse takes note, then goes to smell the nearest flowers.