Herein lies the most important piece of advice you could possibly glean from older and more experienced Dartmouth students: Take Advantage of EVERY SECOND of every MINUTE of RELAXED freedom!
Fondly called Camp Dartmouth, this period of your Dartmouth experience will be characterized by inordinate amounts of (good) free food, ridiculous activities (that quite frequently amount to more free food), opportunities to meet all kinds of people, and lots of socializing. Your only responsibility during this time period is to eat lobster, shake the President's hand, meet people, and unpack your room. Your rigorous academic experience will commence within only a few short days after the official start of classes, so enjoy your full-fledged stress-free FREEDOM as much as you possibly can.
While some may interpret this as party every night, be sure to also take advantage of all the activities. Get to know the campus well before everyone else shows up. Check out the Hood Art Museum. CCheck out the a capella groups. Attend lectures and info sessions. Stop by the hypnotist show. Better yet, volunteer to be hypnotized and become quasi-famous among your fellow 'shmen.
Spend time bonding with your 'floor family'. You'll want to be comfortable with the people you're living with! Don't panic if it seems that everybody has already formed groups of friends. Orientation is a time to meet new people, and there's a good chance that those orientation cliques aren't actually 'best friends' after five days of knowing one another.
Also, make sure that you are aware of Pre-Orientation, the time 'shmen first begin trickling into campus, between when DOC trips end and when orientation starts. Pre-orientation is essentially another 'Camp Dartmouth', only with more freedom, and a less overwhelming number of 'shmen to bond with! Pre-orientation fun includes sometimes bizarre activities like 'capture the watermelon', and scavenger hunts galore. Again, these usually include free food that will taste that much better after your camping stove cooked DOC trip meals. Pre-orientation is prime time to set up your room, and hang out on campus with your trippees before the rest of your class arrives!
There really are good programs. Organizing committees have gone to great lengths to make them interesting, informative and fun, and they have succeeded in accomplishing these objectives. You will also have the opportunity to take aptitude tests to gauge your placement within certain classes on campus. Even if you think you won't perform superbly, it's certainly worth taking the tests. You cannot be placed down if you already have AP credit. Remember that your education does not pivot around your ability to do well on these exams - don't spend all of Orientation studying.
Strike the balance between work and play - if you can learn to do that during your Camp Dartmouth phase, you'll be set for the rest of your collegiate experience.