The fun is decorating the object with real shells, or with cutout paper shells. Many coats of clear varnish will give the box a satin, smooth finish. If you sand each coat of varnish lightly with fine sandpaper the finish will be as smooth as ivory. To create a box or picture frame encrusted with shells, overlap the small shells onto larger shells to create an overall effect. A hot glue gun or Elmer's glue can be used to secure the shells to the background.
Shells also look good when covering the top of a wooden tissue box for use in the bathroom.
If decoupage is a craft that interests you, photo copy real shells and cut them out to arrange and apply to a painted surface, such as a table top, wooden box or frame. Then coat with many layers of polymer medium to preserve. Another use for shells is to decorate a fat candle. For this you'll need a box of clear wax paraffin, the kind used for canning and can be found in the supermarket. Put the wax in an empty coffee can and place it in a pot of boiling water to melt on the stove.