The Advanced Placement Teachers' Resources Search
http://apcentral.collegeboard.com/apc/Pageflows/TeachersResource/TeachersResourceController.jpf is a good place to browse for a range of subjects.
A comprehensive list of books and sites by 1999 physics Nobel Laureate Gerard ′t Hooft is
HOW to BECOME a GOOD THEORETICAL PHYSICIST
http://www.staff.science.uu.nl/~hooft101/theorist.html
and for dry humor one can read his other essay,
HOW to BECOME a BAD THEORETICAL PHYSICIST
http://www.staff.science.uu.nl/~hooft101/theoristbad.html
A good physics forum I have browsed is
Physics Forums
http://www.physicsforums.com/Physics Today magazine is worth browsing. For example, it recently discussed "Teaching general relativity to undergraduates"
http://www.physicstoday.org/resource/1/phtoad/v65/i6/p41_s1?bypassSSO=1Some of these books may be accessible to gifted high school students who have studied AP Calculus BC and AP Physics C.