My oldest niece (10yrs old) is very science minded- This year, I got her a gem mining (geode) kit and some books, which might seem weird but she asked for it. In the past I've gotten her books relating to topics she likes(bugs!), experiment kits, telescopes, DVDs (she loved planet earth), bug hunting kits, and puzzles/brain teasers. I think hers (and her siblings) favorite was the butterfly grow kit we got them, they loved watching the whole metamorphisis take place! However, if you live in a colder climate I would wait on that one so as to not have to release the butterfly in the cold (for us in MN I got it for her birthday in the summer). One time, when we were traveling, I got her a naturalist-styled coloring book that instructed what colors to use and where- by the time the plane landed she could identify most of the animals in the woods! For science minded kids, what I have found works best is to get them things they interact with rather than things they simply posses. Puzzles and activity kits are great at this. Singing elmo doll, not so much.
Most of my gift ideas actually come from our science museum. I volunteer there so when one of the kids birthdays rolls around, I swing through their gift shop. It's always a hit! Along those same lines- maybe instead of a thing you could get them an experience. Take them all to the aquarium or science museum. Part of the my reason for volunteering was that it gave me automatic membership at our science museum. This translates to free admission so we go often now.