in this whole 'to think or not to think about college' debate, i think what people are trying to get across is that you shouldn't think of college as a goal. in the short term, it probably is, but it shouldn't be the focus of your life. with that being said, i definitely agree that mucking around is not a good idea. having fun while staying on track though, is completely okay. and i think you (Siriuss) have a pretty good approach of thinking about college - i fantasized a lot about libraries too.
in terms of what you can do right now... well, i think very few people actually 'planned in advance for the University of Pennsylvania', thought i'm sure most planned in advance for university in general. keep doing what you love and you'll be fine. if the university doesn't see it, it's their loss, also probably meaning that it wouldn't have been a good match for you anyway. after meeting some incoming college freshmen of various colleges, i am beginning to truly believe that everybody ends up where they do for a reason.
for my whole life i was aiming for stanford. and then the summer before my senior year i toured the east coast. i ended up applying for Penn ED and got in. so yeah, you'll be fine.