What a tough prompt! Very hard for me to keep it "plain." Of course first I had to think about what that meant to me. What seemed to be the essential quality of plain for me is something without ornamentation. So I started with a picture of plain white items.
That led to the thought of the potential of something "plain" to be decorated or added to. Vanilla ice cream came to mind! And then I had to start thinking about the need to decorate things as one of the reasons why we consume so much as a society. The next thought that came along was my goal that all my responses to art prompts include something recycled or reclaimed. Where to go with this? Ummm...that would be to embellish something. In this case, a plain old glass jar I've been using for a pencil/pen holder. Of course, I also had to add a little vintage text, "delights you can make...."