Off te top of my head.
1) Do not fly in the day of the cruise. Arrive at least a day early.
2) The Disney transportation to the cruise is super easy. And if you are the least bit worried where to go, then take it. However, if you use the Disney transport, it gets you there whenever it gets you there. You will make it on time to get on board, but you may arrive at 3:00 PM for a 5:00 PM sailing. Meanwhile the ship started boarding around noon. The earlier you arrive at the cruise port, the sooner you get on the ship, and the more time you have to enjoy the ship. You also get first crack at the excursion desk and those elusive Spa bookings. A 3 day cruise is short, you are going to want to get on that ship as early on your departure day as possible. Just remember to pack a carry on bag for your child.
You basically have three ways to get to the cruise port.
1) Disney transport. Easy, but may get you there later than you want.
2) Town car. Easy, gets you there when you want, but for a family of three, more expensive than the Disney transfers.
3) Rental car. Cheaper than Disney transport, gets you there on your schedule. But you have to know where to drive to. You have to pick up the rental, drive a one way rental to Port Canaveral, drop the car off, and take a free shuttle from the rental place to the Cruise port. Not difficult (I have done it) but with a 17 m/o maybe not the most fun.
choices ... choices.
More advice. Room service is free (except for room service beverages) if your child wants to nap, take advantage of the room service (heck, take advantage of it anyway). You should tip the person who brings it though.
As someone else said, see the shows, they are top notch.
Check out the little used deck on the bow of the ship. I think it's off of deck 6 and 7. Not many people know its there. Supposedly, if there is a wedding, that is where it will be held.
-dave