I always have a copy of Emmett's brilliant Gear Bearing out on the coffee table - it's such a great talking point and introduction to 3D printing - and recently someone I showed it to suggested it might make an interesting bangle. I'm not sure about its fashion-worthiness, but as a printing challenge I thought it was an excellent idea.
I had already used an idea by Sal to print gear bearings with large circular holes instead of small hexagonal ones (http://www.thingiverse.com/make:26306), so I just took this up to the next level. I have posted the modified .scad file so the design is still fully parametric, and also a copy of the STL file that works with the tolerances on my printer.
Here is the video of the bangle working as a gear bearing (but not as a fashion item!) http://www.youtube.com/watch?v=Qt39vLo5l90

1. This is a large print and will use a lot of plastic. I strongly suggest printing a smaller gear bearing first to ensure you have the right tolerance for your printer.
   Failure to do this may result in your bangle becoming a fused lump of plastic that doesn't move at all.

2. It is critical to choose the right slicing tool for preparing your gcode. Until recently I thought that Kisslicer did the best job of avoiding linking the gears to the outer frame but I have now discovered that the Split option in Slic3r is even more reliable.

3. Print any way you like.

4. Unless you have chosen a very large tolerance gap (I used 0.15 mm) you will probably find there are still places where the gears have fused together slightly. Wiggling and rocking the parts will free most of these up nicely but I had a couple of really stubborn ones that wouldn't budge. The best trick I found was to used a strong penknife blade slid between two teeth and then twisted to lever the cogs apart. Once all the planets are free, make sure it turns fully through several revolutions.

5. Running in the gears. Find something large and round that will fit tightly inside the bangle (I used an old polycarbonate measuring cylinder) and press fit it in. I would not recommend using your hand inside the bearing at this stage.
   Turn a bicycle upside down and get someone to turn the pedals for you while you press the outer rim of the bangle bearing against the moving tyre and run it continuously for several minutes. This should smooth out any bumps and give you a nice free running bangle bearing.
   And watch out for the teeth - these things really bite hard!