Will google sketch up do? (i have not tried google sketch up so let me know
)
Or
Have you tried the Softimage Mod Tool, it has a built in terrain maker. Obviously you can't use it commercially but for testing it would be alright.
Or
T.Ed is brilliant for making terrain and for placing buildings, it does cost around £18.
Or
You can code the entire world into it by making a heightmap and colourmap. One of the tutorials walks you through this i believe under the name "Terrain"
Here are some terrain makers that i have found:
http://freeworld3d.org/
http://www.earthsculptor.com/
http://www.pnp-terraincreator.com/user.php?section=License
(this one looks really good because you can make maps with the free edition and still use it commercially. the link is to the comparison of the different sorts you can get)
BJKDAW