The only things you must plan for a Disneycruise are:
1. Get your paperwork (passport or Birth Certificate+photo-id are mandatory, it is efficient, but not mandatory to print and sign boarding papers in advance, they can be printed at boarding). Just to be clear: no id=no cruise, so triple check that this is in your carry-on luggage!
2. Fly in the day before your cruise to have enough time to counter flight delays/cancelations
3. Board the ship before "all aboard" time (duh!)
4. Attend the muster drill (the only mandatory thing on a cruise)
5. Relax! You are on a cruise and the point of a cruise is to RELAX and have a good time with your family.
Done!
Now, you will want to study the Personal Navigator to check out the activities on the ship. As you will be sailing a 3n Dream cruise many Navigators can be found here to prepare:
http://disneycruiselineblog.com/personal-navigators/ on the "Disney Dream" tab.
A 3n cruise is way to short to do all the things the Ship and cruise have to offer, so don't even try to do everything as it will just lead to major disappointment. Accepting this will improve your cruise experience and inevitably lead to the last thing on your to-do list:
6. Visit the "Future cruise desk" to book your next cruise with On Board Booking (OBB) benefits.
Enjoy!