Flatland Mod cleanup terrain and useless entities. Flatland Mod initially ask you what type of terrain you want to use : grass (for a natural look), concrete (permit to see correct blocks over water and a grid) or nothing.
This also remove all decorative entities, dead-trees and stone-rocks.
You can still use generation settings to have resources, enemies and water.
If you want a clean world, set everything to minimal in world generation settings