I did The Young Scientists Club with my older boy when he was 7 and 8 years old. They were simple experiments they would mail you once a month- we made a paper mache volcano that blew up; we made crystals with suger; magnets. Then we did some of these Edmund Scientifics chemistry experiments.
He did just now an online Germ Course through Northwestern GLL, which was fun, but I think we are just too busy for online courses. We'll get back to chemistry now!
However, one thing I've noticed is that he is really, really excited about science. He knows ALOT about science, and he is obsessed if we go to a science museum. He tells people he wants to be a doctor (like me!), and I've gotten compliments from parents at his school how much science he knows.
So I figure at least THAT has been successful, at stimulating his interest in science.