I found a way that you can create custom tools for sculpting in CAW. In the UserToolData folder, there's a subfolder called TerrainSculptBrushes, and in that is a file called TerrainSculpt.ini, which has data related to ll the sculpt tools. You can copy and paste any of the codes in there to create a modified version of an existing tool, or you can even use your own .tga graphics. Now you won't even have to manually sculpt the details on your hills and valleys bit by bit, they'll be there automatically and you'll just have to fine-tune then.
Tutorial on creating custom terrain brushes:
Go to Documents > Electronic Arts > The Sims 3 Create A World Tool > UserToolData > TerrainSculptBrushes
Open the file "TerrainSculpt.ini" (will probably just appear as TerrainSculpt)
Add the following code to the end of the file, and change the word "Custom" to the name of your new tool:
Name = Custom
WiggleAngle = 0
ComponentCount = 1
; First component
Comp1_Op = 0
Comp1_Texture = Custom.tga
Comp1_BaseTextureValue = 0
Comp1_AlignWithMoveDirection = 1
Comp1_Strength = 0.01
Comp1_Width = 1.0
Open up any of the existing .tga files in the TerrainSculptBrushes folder in a graphics editor that supports them. I personally use GIMP. I assume they works with Photoshop, but haven't tested it yet since I don't have Photoshop.
Blank out the content so the image is pure black.
Create a new 64x64 image (When I tried to edit the .tga file directly in GIMP, some features, including blending with the paintbrush tool, didn't work properly, I'm not sure if this happens in Photoshop).
Create a grayscale image. When applying the custom tool to the terrain, the darker parts represent lower terrain (black being no change) and lighter parts represent higher terrain. For my custom tools, I just dab the middle using the paintbrush tool with a different brush for each tool.
Select all then copy the image.
Paste the image onto the .tga image, then save with the name used for Comp1_Texture in the TerrainSculpt INI file.
Now you can try out the tool in game. :P
I haven't tried making a tool that lowers the terrain yet, but if you want one, I assume
you can do this by inverting the .tga image and setting Comp1_BaseTextureValue to 255, but I haven't tested it yet.