If you are working on a 2D game make a mockup or “fake screenshot”
If your game is 3D make a visual vertical slice: a small part of the game, not playable, that sets the mood, the visual feeling, the atmosphere and the tone of the project.
Either delivery should set the target in terms of production value and visual polish.
Base it on the art direction discussion you had today.
This delivery won’t likely involve the whole team. The other member should continue on the previous tasks: level design (greyboxing), narrative design, programming…