Coming from Unity2D, which world building tools do you recommend?

Hi im currently working on a 2d title which has minor 3d elements.
For a new project id like to learn the 3d side.

Which tools / tutorials / plugins would you recommend?

For example making terrain, rocks, trees, rivers, bridges, streets, houses, clouds etc

It doesnt need to be high poly count. But it like to be able to make a small map, with some trees, flowers, gras, a river, a few cardbox houses. A fence etc..