I usually use flat models to decorate cards and scrapbook pages, or create scenes. Modulars are perfect as hanging ornaments, and some of them can even be used as vases for origami flowers, like this one:
The modular in the picture above is called "Ore", and was designed by Toshikazu Kawasaki. The diagrams are available in his book, Greatest Dream Origami (ISBN 9784255005058). It consists of six units, that are very simple to fold and assemble.
The flower and the leaves are a variation of the traditional lily.
I used 8" (200 mm) squares of coordinating scrapbook paper to fold the models.