Three (okay, four) favorite theme trees:
In 2001, I made a NYC-themed tree with red sequin ball "apple" ornaments and yellow taxi matchbox cars. I topped it with a Statue of Liberty. Very poignant that year.
Another year, we had a tree for swingin' lovers. Kitschy aluminum tree with a leopard print tree skirt, and I made martini ornaments for it. So Shag.
A couple years ago, I was working on a themed tree for an art auction for a local AIDS charity. It was...well, how do I describe this thing? The theme was Studio 54. I started with dowel rods and made sort of a feather tree/bottle brush tree by crocheting pink eyelash yarn to the boughs. Then I added disco balls and silver spoons. I made ornaments of wooden disks with the 54 logo on one side, and black & white (with color accents) photos of the 54 "in" crowd on the other. Andy Warhol, Jerry Hall, Liza Minelli, etc. I covered them with a layer of glue and sprinkled tiny clear beads over them to give them a textured finish. I hung them on the tree with silver ribbons. The topper was the famous Studio 54 icon of the Moon snorting a spoonful of coke. What was I thinking??
Oh yeah, the last one. One year at work, someone plunked a small tree on my desk and said, "Decorate this for your department." Erh? I had a few ornaments, but it didn't really stand out. I started going through my desk drawers looking for something to make people remember it, and I came across all the little desk toys and action figures I'd been collecting forever---Happy Meal toys from various Disney flicks, Dana Scully from the X-Files, a knockoff Teletubby whose eyes glowed creepy red when you pushed a button on his tummy, Herme the elf who wanted to be a dentist... So I stood them all around the tree with Scully putting the star on top. I think our department won a prize for that one.