Amateur Astronomers Association of Pittsburgh
This group often holds public "star parties," where more experienced members aim their telescopes at points of interest, and guests can come observe. They also provide free online astronomy resources (such as a guide to weather conditions and upcoming celestial events).
BrainCake from the Carnegie Science Center
BrainCake.org is an online sisterhood of girls ages 11 - 17 featuring programs, scholarships, virtual mentoring, girl blogs and much more. Check out the "happenings" section for Pittsburgh area events, share your ideas on gender equity in "We Hear That", learn about awesome women in STEM in "She's Livin' It" and check out the way you can get involved as a BrainCake girl!
Carnegie Museum of Natural History
The CMNH website is full of opportunities, from a summer camp to free public events to a list of workshops and field trips your teachers can schedule.
Carnegie Science Center
In addition to the exhibits and activities you might encounter on a normal day, the Science Center offers summer camps, sleepovers, and an ever-changing list of workshops.
CS2N - The Computer Science Student Network
This initiative from Carnegie Mellon University encourages students to engage in robotics, animation and computer science activities for fun and competition.
NASA: No Boundaries
This is an annual contest sponsored by NASA, in which groups of students research and promote a STEM career to their peers. Winners receive cash prizes and a VIP experience at a NASA center.
The National Aviary has a summer camp, and a wide variety of workshops and events you can attend. The Aviary's website also includes information about local birds, and projects and activities you can do at home.
Pennsylvania Space Grant Consortium, k-12 Education Opportunities
This site lists many space-themed contests, programs, and activities, both within Pennsylvania and at the national level.
Phipps not only offers classes and trained volunteer opportunities; they occasionally have paid internships for high school students.
Pittsburgh Zoo and PPG Aquarium
The Zoo has half-day and full-day camp for ages 2-13, as well as a variety of internships and educational programs for teens.