Elementary through High school:
YOSA Camp - The Youth Orchestras of San Antonio operate during the school year, but there is also a special two-week summer course, geared towards orchestra playing.  There are also music theory, composition, and strings courses.  I've had the opportunity to teach at YOSA's summer day camp for 2 seasons.

High School, advanced playing only:
BUTI - Boston University Institute - offers an intensive chamber music course, and a longer orchestra course.  Students stay in dorms on the Tanglewood grounds.  There are lots of opportunities to hear the Boston Symphony and work with leading orchestra and chamber musicians.

Quartet Program - The Quartet Program is Charles Castleman's pet project (my teacher), designed to provide a focus on chamber music and solo playing.  

New York String Seminar - Jaime Laredo's full-scholarship, 8-day string orchestra festival, over the Christmas holiday.  Ages 15-22