This one is somewhat sentimental. Last year I had a tonsillectomy and because my sister was given a kaleidoscope when she had her tonsils out (she was four), my mother went to quite some trouble to find me a kaleidoscope as well.
Kaleidoscope making kit
It really is quite fascinating how simple the design is. It's very easy to make, just three reflective plates arranged inside the tube and when you look through the eyepiece, whatever you're looking at is reflected again and again and again.