(1) Don't waste time in Computer Science / Engineering when you hate programming, because you'll get horrible marks and wish you hadn't dropped 40% of your classes in second year.
(2) Don't let your high school guidance counsellor talk you out of being a science major, and spend first year taking random electives and feeling like you should have went to CC instead.
(3) You're actually really smart. Apply for that research position and scholarship, because you'll get both. Stand up tall, be proud of yourself, and don't avoid trying anything for 2 years just because you feel like you probably won't succeed
(4) Where's your community service at?
(5) Take pre-reqs for med school early - you never know what you'll want to do later
(6) When you're away on study abroad, stop studying for the MCAT and enjoy your life. See the world. An extra semester is no big deal.
(7) The things you didn't plan to do, weren't sure of, and didn't think about as "resume builders" actually served you the greatest, later on. Who knew?