First, start with something that will not get damaged or destroyed by rain: this could be the lid to a jar or a tin of snuff (for an interesting effect, you could either put your design inside the lid or you could run over the lid with your car to get a flattened, crumpled look); a sand dollar or shell; a piece of wood painted with exterior paint; a piece of glass with smooth edges and a hole in the top, etc.
If there is no hole already, drill one at the top of your piece. Use copper wire, which you can buy from your favorite hardware store, to hang it by. Copper wire is strong and malleable, so it’s easy to work with and will withstand wind and rain. Leave the wire fairly long so that you have plenty to work with; you can always snip the excess off later with some wire snips.
Play with your ideas for decorating this piece until you’re satisfied with your design. You can use small stones; glass, metal, or plastic beads or buttons; pieces of crockery or tile; metal doodads; bottle caps; safety pins; small resistors; pop-tops; old keys; anything your creative, fertile mind can come up with.
Once you like your design, use exterior grade clear caulking to adhere the decorations (some caulking is white before cured and it turns clear as it dries; but it will say “Clear” on the tube). For small pieces, the easiest way to apply the caulking is to squirt a small amount onto a piece of cardboard or scrap wood, then use a toothpick or cotton swab to apply the caulking to the back of the decoration. Take care not to get the caulking on you or anything you don’t want it on. Have a damp rag or sponge handy to wipe up any smears.
The decorations should be firmly adhered within an hour, but I always like to let mine set overnight before hanging.
Another option, for a temporary ornament, is to use materials from the out-of-doors such as twigs, seeds, pine cone bracts, bits of greenery, etc., and use copper wire or twine to attach the pieces to one another.
Above, in the center of the image, is an ornament I made as a memento for last year’s Holiday Treasure Hunt, using a rusted top to a can*, a Chinese coin, a stone cabochon, a metal button, silver beads, and transistors from an old, discarded computer motherboard. These ornaments can of course be hung indoors as well as out.
Have fun!
* Anyone working with rusted metal be sure to have your tetanus shot up-to-date! Use good sense when working with recycled products, and of course, play at your own risk.