Fitting weather vane: http://www.thingiverse.com/thing:42049

This thing is fully parameterizable and meant to be customized before printing. It is optimized for support-less printing (blades are oriented vertically) and designed out of two parts so that even on small build-space printers pretty big propellers can be made. The appended *.stl file is only an example. Customize the model to get the biggest propeller you can print (almost twice the diagonal of your build-platform length).

One could print a lot of them as toy propellers in different colors all with different random parameters in reasonable margins and mount them on a kind of "wind sculpture".

Ideas for improvement:

Right now its just a toy but one could:

• add grooves for ball bearings
• twist the blades
• generalize to 3 or more blades (4 non-coplanar should be easy)
• alter the blade shapes to something less classic but more fancy e.g. modify the blade to a more aerodynamic shape (NACA4)
• make the blade-angle adjustable
• make a coupling for a dynamo

Going crazy:

• add button-cell powered laser-pointers to get a free laser-show at squally new moon nights
• mount (suitable!) whistles to the tips (so your child wont be overheard when cycling), but don't forget the (slide-able?) muting plugs then.

Print diagonally on build plattform for maximal size.