Old keys may no longer be useful, but they still look cool. If you've amassed a box of old keys you really don't need, but don't want to throw them away, use them to make crafts. The more old keys you have, the more impressive your key crafts can be.
Use keys as charms to create one-of-a-kind jewelry. Attach a small or lightweight key to a metal ring and loop it onto fish hook earring wire for a retro-'80s earring, or string keys between beads for a funky necklace. Keep in mind that keys will face sideways when strung unless you use a metal ring, which will allow it to lay flat. If you know how to solder, keys can be fused together and bent into cuff bracelets or necklaces, or they can be hot glued to a blank metal cuff.
Decorate your home with old keys by stringing them along your walls. Take a long piece of string or fishing line and thread a key through its hole 12 inches from the end. Tie the line in a double knot around the top of the key. Repeat with as many keys as you like, but if you have dozens and dozens of keys, make several garlands, rather than one super-heavy one, and hang the garlands in layers. Make a small loop on each end of the garland. To hang, nail a brad through each loop.
Use larger, older keys as wind chimes by hanging them from a metal or plastic hoop made from a cross-cut coffee can or plastic 2-liter bottle. Drill holes in the hoop all the way around, and use fishing line to attach the keys, tying double knots around both the keys and the drill holes. The fishing lines can have slightly different lengths for a staggered look, but don't make them so different that the keys won't knock into each other in the breeze.
Hand sew your old keys onto to a purse, belt or jacket. Hold the key in place and push a threaded needle through the hole. Loop the thread through the hole several times to secure the key. For heavy material such as leather, you'll need a heavy needle. Add one key as a singular embellishment, or cover an item with keys. Keep in mind that the keys will add weight to your accessory if you use a lot of them.