I love Nebel's Building Foundations of Scientific Understanding, but he is probably a bit old for it since it is geared more toward younger kids. I wish that he'd write a sequel for older kids!

You might want to give a try with some of the lesson plans from the College of William and Mary Center for Gifted Education here: http://cfge.wm.edu/Curr_listing.html#Science

I have a bunch of free online science resources on my website here as well: http://www.galileoeducation.org/Homeschooling.html

Study Strategies for Accelerated Learners