Apart from what DG and TrueBlue have posted, there is also something to be said about the exam being an exercise in futility, if 200 out of 300 people end up giving the exam. The entire point during summers, or at any stage in the recruitment process for that matter, is differentiation. How you do it, and to what extent, is largely up to you.
My point of view, of course...
Besides, live it up as much as you can right now without worrying too much about these details....once you get to Cal, it'll get tough. I can say, mostly on account of profound and painful personal experience, that the environment will be of the sorts few would have been exposed to earlier.
You'll have to work hard, but remember that this is NOT all you should be doing. C will offer you a chance to build yourself up, to round your profile... No dearth of clubs, sports and extracurrix.
The only thing that'll pinch you, is lack of time.
At any rate - it WILL be the best two years of your life. Money back guarantee.

~K