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.
By Cay from Green Cove Springs, FL
I have many old keys and thought this would be a great way to use some of them. I'm having a problem visualizing this though. Is there a way you could post a picture of the one you have?