I always love anything with a woodland aesthetic, and this moss and mushrooms arrangement from the new April issue of MS is lovely. It is so simple and fresh, and sometimes I think that less "stuff" in an arrangement makes for a natural elegance.
Assembly? So fast--this is the perfect quick centerpiece for a dinner party. Using a container of your choice (I'm thinking either very simple like the one above, or one out of a very nature-inspired material would be beautiful. Birch bark paper? Faux bois?), line with plastic if it is not waterproof. Set a mound of chicken wire or any other stiff material to make a puffy base for your moss. Set moss on base material, insert a few sewing pins to hold in place. For mushrooms, it would be fun to use a variety of types, all available a your local grocer. Just insert a toothpick into the base, leaving about 1" stick out. Insert that end into the moss, mist, and enjoy! The moss will last weeks with regular misting, you'll need to switch out the mushrooms every few days.
But while you have that beautiful moss base, why not switch it out occasionally with other woodland accessories? Little birds with speckled eggs, a few blooms in a flower vial...let me know what you come up with!