How do I render multiple times to have an inventory UI in 3D?

I want to render the world and after that an 3D-inventory using different render settings. I do not want to render to a texture. Is that possible? Is there a concept of multiple "worlds" or being able to render multiple cameras?

A similar problem is rendering the weapon and hands in a First Person Shooter. You want to have them camera-relative and have different rendering settings.

You can use multiple cameras, combined with layers. For an inventory, the inventory cam would have a higher depth than the normal cam, and would have clear flags set to depth only, so the other camera can show through the empty spaces. The culling mask would be set to only render items on the inventory layer, while the other camera's mask would be set to render everything except the inventory layer. Rendering a weapon in a FPS is the exact same idea.

Not sure whether this is exactly what you're looking for, but take a look at the documentation here under Culling Masks.

You basically add another camera to your scene. Also add your inventory or weapon to the scene. Put these items (inventory or weapon) on a separate layer and change your cameras culling mask to render only this layer. Also exclude this layer for from being rendered by your normal camera. Finally, set the GUI camera's clear flags to a higher number than your normal camera to make sure your inventory/weapon is always rendered on top of your scene. That should do the trick.