Any of the built in effects. I use that term broadly. I'm talking about any of the noises, adjustments, erosions, so on and so forth to shape a landscape.
If you have a file you are using, like a DEM or something, load it using the file node and just shape it how you want using the things listed above. I can't give you a direct way of doing things, because every build is going to be different. SO I have to be a bit generic unless you want something extremely specific.