What are ideas for a nature cover?

So for a school project we created nature notebooks, where we went out and drew/wrote observations of nature outside. We have to put it in a notebook and we need a front and back cover. The front needs to say 'Nature Notebook' but the back doesn't have to say anything, though it still needs some decoration. Any ideas for what to put on the front of the cover that's nature related? I know some people made the cover out of a piece of tree bark and some people framed the cover with twigs around it. Any ideas different from these? Thanks!