<p>In my opinion, if you’re shooting for the stars, you have to figure out what the stars are exactly first. Dream of going to a specific selective university? Look up what sort of summer programs they have to offer. Once I read a bunch of books, visited a campus, and talked to a bunch of people, I now have a good idea of what they are looking for. They don’t want the A+ student who has no clue what they want to do and no unique extra-curriculars. They want passionate, driven people who don’t live out of textbooks. Exclusive colleges want a diverse student body. They want to be able to look at your application and say “hey, this is something we haven’t seen before.”
In my opinion, if you can afford to go to an academic summer program of some sort (or get financial aid), go for it! You can learn so much about what you want in a school. You can also meet the student body and see what kind of people they are. Are they entrepreneurial? Talented? or just an A+ kid who tries ridiculously hard in high school but has no passion?</p>
<p>Gotta find your passion…find what interests you, then fly with it. Now is the time to immerse yourself in the wide variety of different activities/subjects in this world and see what you like best!</p>
<p>P.S. to answer your question more specifically, I think a combo of all three would be the best. Have a few volunteer organizations whom you are involved with for many years–don’t go on a summer trip to Africa type thing–the former shows you have real heart, not tons of money (like the latter). Internships show that you are a strong competitor and have talent; summer programs show that you are really serious about that particular university and hey, wouldn’t a professor recommendation letter be worth every penny? If you can, do all three 
it’ll be worth it in the end!</p>