You will need:
Hot glue gun
This ornament idea came about primarily as a way to deal with an excessively large seashell collection, most of which never saw the light of day. You may have such a set as well, particularly if you have been given one of those little baskets of seashells that seem to be a common bathroom decoration.
Cut five-inch lengths of string, double them, and knot them. Glue them into the open end of the shell so that it hangs point down; in the case of scallops, glue unobtrusively to the hinge area. Tie a ribbon bow around the string where it meets the shell.
If you have a number of spiral shells of the same type and size, you can make stars. Arrange them on a table, open ends facing in, until you know how many will fit together. Carefully apply hot glue around the open edge of one shell and hold it next to its neighbor until the glue cools. Continue with the rest of the shells until the star is complete, gluing the loop into one of the joins. Tie ribbon around the loop.
Sand dollars are also fun, though fragile. (They are best found on the beaches around San Francisco in October if you can stand the cold; leave them in a bucket of diluted bleach for a week or so, then dry. Don't worry about getting all of the sand out. You can't.) Glue a loop into the hole on the backside, then anchor it with a spot of glue on the edge of the shell so it hangs straight. You can also use craft paint to follow the star patterns on the front; I suggest diluting it so that it looks like watercolor.