I've been going to Jnr Scientia/Scientia programs for 6 or 7 years now. I've stayed residential (on campus), gone to camps and taken courses in like 5 or 6 different fields eg medicine, law, languages, fine arts etc. the courses are a great way to meet new friends and see what you're really interested in.
I've found my interests changing and that helps me to pick my subjects for electives and subjects. It also helps you to figure out what you like at uni, and you get to use all the facilities there. I've been to dissection rooms at UNSW with other med students and the faculties of law, bioscience, engineering, the different libraries etc
The GERRIC programs are great to help you experience the different uni courses available in a way suitable for ppl who are non-uni age. If you ever get the chance, go to one, it totally changes your view of uni and subjects. Try picking courses that you think you might be interested in but wouldn't usually pick, often you find that you are more interested than you thought! Once again, if you get a chance, GO!!