For my niece's 15th birthday today (fearsomely gifted at everything but especially maths and science), in addition to cash I gave her a 'Feel Flux.' This is a 'toy' that is a great demonstration of Lenz's Law. Added to that it really is a work of art and beautifully made.
With my nephew's birthday in 5 months, what scientific gadget/toy can you suggest? Nothing that he could harm/kill his sister with!