I have to admit that I don't do a lot with Terragen.
I wish I had the patience to really learn it and then to go on and buy a commercial license but it's really not in me to spend a lot on graphics software plus, waiting for things to render is a pain.
I'm sure it's faster now but those early releases left a bad taste on my internal clock.

What's here is from the free version released a few years back (and no longer installed on my computer so I can't tell you which version that was). Now called Terragen Classic,  it still has a non-commercial (free) option.
I walked through some of the tutorials that I found around the web (most are no longer there) and came up with these images.
You can see that with a minimal amount of effort and no extra plug-ins, you can still get some pretty nice pictures.
If I can find the links to the tutorials, I'll include them.