Background elements

So I've been using background elements in my recent illustrations and I've had a lot of trouble just thinking of what i can add to it without just creating clutter and the answer seems to be pretty simple - with a simple idea of semiotics and to keep the items relevant.

For example, if i want to show that something is outdoor then i could perhaps add a tree, grass, clouds for a example to fill the top half of the page. Mix this in with some more abstract shape relating to the topic and it starts to create something much more interesting yet still make sense and feel part of the illustration.