1. Just plain Legos
These are increasingly hard to find - most of the boxes on the Lego aisle at the toy store are themed building kits revolving around superheroes, Harry Potter, or The Hobbit. Skip the directions-included, no-imagination-required themed sets and go for the plain old bucket of Legos, which has endless building possibilities and just might accidentally inspire some creativity in your child.
Unless your kid really likes looking at blurred smears, you'll probably want to avoid cheap plastic models and save up for a sturdier model - look for the words "rack and pinion" in product descriptions to make sure you're getting a microscope that'll be easy to focus. Throw in some blank slides, too, because I'd be willing to bet your nine-year-old's fingerprints, boogers, and tongue-print will be far more fascinating to her than anything that might come mounted as part of a standard slide set.
3. Minimalist 'Women in Science' t-shirt
Artist Hydrogene created a line of minimalist designs showcasing the groundbreaking ideas of female scientists, including Marie Curie, Jane Goodall, Rachel Carson, and Rosalind Franklin. You can get these beautiful designs printed on t-shirts (including kids' sizes) as well as posters, and I have to stop looking at these now before I go on a Pokemon-style gotta-catch-'em-all rampage.
4. Root Vue
For the budding young botanist, why not give the gift of a green Christmas? Root Vue is probably the best DIY grow-kit I've seen, because it lets your little gardener take a peek at what's happening under the soil - a trick that should help prevent him from uprooting those tiny carrots to see if they're ready yet.
5. Giant Microbes
Giant Microbes are adorable plush models of disease-causing bacteria and viruses, human body cells, and all kinds of other creepy crawlies. (Depending on how old your child is, you may want to steer clear of the selection of venereal diseases.) Each of the 50-odd different options comes with a card explaining its biological background, and they are equal parts cute and curiosity-sating.
Your child can play with toy rockets ... or build and launch her own! Building rockets with my grandpa was definitely a childhood highlight; don't forget the extra engines for future re-launches, too.
7. Ice cream maker
Anyone who tries to tell you cooking isn't chemistry is dead wrong. Teach your kids about the scientific principle of freezing point depression while scarfing down some tasty homemade vanilla!
I haven't figured out a way to make adding sprinkles and chocolate chips revolved around a science-related theme yet, but when I do, you'll be the first to hear about it.
8. DIY robot kit
Learn about circuits the best possible way: by making your own robot*. Maybe I wouldn't have fared so poorly in my college electronics-and-magnetism class if I'd spent more time as a child trying to create adorable artificial life like this.
(*Science mom cannot guarantee toy robots will follow Asimov's Three Laws of Robotics, so buyer beware.)
9. The Private Life of Plants
Pretty much everyone who owns a DVD player already has copies of Planet Earth and Blue Planet, but here's a series on naturalism you might not own. Sir David Attenborough is one of the few people in the world who can make botany interesting to me, and he does it so well that I've watched every episode of this show four or five times apiece. Watch as Sir David rhapsodizes about flowers! See him recklessly endanger his own life! Watch as wasps relentlessly hump wasp-shaped flowers!
Note: This was made in the UK, so be careful to buy the right region of DVD for where you live.
10. Scientist bobblehead dolls
How cool is it to have Marie Curie benevolently watch over your high schooler while he works on his Chem I homework each night? "I know stoichiometry is hard," her wise visage says. "But at least working on it won't give you leukemia. Ahem."
Bonus: A gift for the Science Mom or Science Dad
After Mom and Dad have cleaned up the dirt from the herbarium, put away all the rocket-construction tools, and stepped on approximately eight million tiny Lego pieces, they deserves a chance to unwind, and they can do it with the Chemist's Cocktail Set from ThinkGeek. Buying a set like this is cool, science-y, and much less likely to kill your parents than making your own cocktail set out of glassware poached from your school's chemistry lab - just ask Madame Curie up there about her radioactive cookbooks.