Also for the record I think I will post this later in the summer or early fall because I want to make the cactus one for a friend.
Anyway, most of these are simple. There are a lot of more intricate ideas for people with more colors, skill and time, but with a few Painters Sharpies and clean rocks, I think most of these will do.
The first image is an example of something pretty complicated that I personally dont have the skill for: