Almost every college I looked (Ivy's as well as private and public schools) at offers summer programs courses for both credit and noncredit purposes for high school students. They involve everything from base subjects (biology, chemistry, etc) to med focused programs. They generally last from 1 week to 4-6 weeks.
These programs are selective however and involve an application progress much like the colleges themselves (transcripts, letters of recommendation, sat/act scores).