I don't know if it was the most appropriate present to get a six year old, but it may have been the best:
Human Anatomy Coloring Book
My nephew just had a tonsillectomy and was kind of fascinated by the process. I was his first post-surgical call to tell me he was OK and loved learning all about science. He was coloring tonsils, arteries and veins.
My brother may or may not remove the 'r-e-p-r-o' pages, but I may have talked him out of that.
Am I a cool aunt or what?