Hills typically aren't just grass, vegetation can be used to block player's visibility as well as give a better look for our environment:
Standard AssetsEnvironmentSpeedTreeBroadleaf
and select Broadleaf_Desktop and then Add.15
and then with our new tree selected, paint the areas where you'd like to see trees in the environment. One thing you could do is paint over all of the land areas and then remove the trees that are along your path.If you hold down Shift and paint, you will remove trees instead of placing them.
As you can see, the world is already much more nice to look at!
If you want to add even more detail to your levels, you can add additional trees and/or materials to the area as long as it makes sense to be there (not adding them in water for example).
You can also make trees of your own using Speed Tree within Unity. For more information on this, check out http://docs.unity3d.com/Manual/tree-FirstTree.html.
For more information on the Terrain engine that Unity has, please visit http://docs.unity3d.com/Manual/script-Terrain.html.