Purchase an inexpensive Styrofoam cone, actually more of a pyramid, at a craft store. Get one about 8-10 inches tall, that has sides (or larger, if you have a lot of old keys and only want to make one).
Paint the old keys gold. Painting only one side is okay. If more color is needed, use metallic paint of additional hues for some/all of the keys.
The natural sparkle of the Styrofoam seems to lend a bit of a festive air to the decoration, so no paint is needed on the tree itself.
Using straight pins, stick one where you want the top of a key to go, leaving about 1/4 inch sticking out. Hang a key on this, then use another key to prop the key at whatever angle you wish. Keep doing this until you have created the look you desire. Leaving blank spaces between keys gives an attractive look.
You can also made the trees in several sizes with the corresponding keys: small cone, luggage keys, etc. Also, varying the sizes of keys on the larger cones is an option.
I have one that I made years ago, and it still looks great!
By varying the Styrofoam shapes you could also make other decorations, such as cylinders for faux candles, cubes, etc.
Source: A craft magazine back in the 70s, I think.
You can use a Dremel (or similar brand tool) to both cut the "teeth" section off the key and engrave information on the large section. Similar to a dog tag. You can then use these "tags" for identify pets and belongings (luggage perhaps?) Small key rings are available at most hobby and craft stores, as well as Walmart.
If you know of any small kids who live in a house or apartment that doesn't have a fireplace they may be concerned about how Santa Claus will get into their house. Go on the internet and do a search for "Santa's Magic Key". You can use any kind of key but the larger the better. You run Christmas ribbon through it and print out one of the poems that comes with the instructions. I print out the poem, laminate it, punch a hole and run it through the same ribbon. You may spend about a dollar per key, and you can sell them for up to $5.00. I've been doing this for a couple of years and I always sell a few and give away a few. Good luck!
Drift Wood and Fishing Line
I made a windchime from some "unknown" keys. I used old house keys, trunk keys, lock keys, etc. I put with the lighter, smaller ones at the bottom of each row and them tied at different levels with fishing line. I hung them from a small piece of driftwood. I used small eye hooks to hang 5 rows of 4 to 5 keys, and 2 larger eye hooks at each end on top of the driftwood. Love the other ideas here too!
Pop Bottle Wind Chimes
Use the top of a 2 liter pop bottle and cut it where to top of the label is. Then poke some holes and hang the keys from the holes. It's a good craft for kids.
I once saw a small box covered with glued on keys. It was really cute. Maybe paint the box black so the silver and gold keys really show up.
Make a key windchime.
Hot glue them on a picture frame.
Make earrings and necklaces out of them.
Many years ago I saw in a movie magazine a lovely bracelet made by an actress. It was made of a variety of keys, it was very "antique looking" and was really attractive. The bracelet itself was a chain to which the keys were attached.
Keeping Drapes Straight
If you want to keep the drapes and curtains straight - put keys in the hem.
Old Key Necklace
I made a necklace out of part of my collection of old keys. There is 11 keys on the necklace with a one inch space between each key.