I'll try not to repeat too much of what others have posted, but since this is the budget board, there are a lot of costs involved with cruising that, as first time cruisers, you might not know about. We haven't sailed on Disney, but have sailed on Carnival, Princess, RCL, and Celebrity.
If you don't have passports, you may as well order them now. They're good for 10 years, but they take a couple of months to get. A traditional passport book will run about $150 per person, or you can get a passport card (looks like a drivers license) that's good for Canada, Mexico, and the Caribbean for about $65 per person. If you already have a passport, make certain it isn't expired.
As some people have mentioned, Carnival, as well as most mass-market cruise lines charge extra for soft drinks. Thats $1.95 per can, plus 15-18% gratuity automatically added to the bill. On the first day of the cruise only, you can generally buy a special sticker for your room key that allows unlimited soft drinks for about $5 per day per person. Disney includes soft drinks at no extra charge.
Gratuities for various people on the ship will automatically be added to your account to the tune of about $10 - $11 per person per day. (Carnival definitely does this and I assume Disney does this as well, but I'm not certain.) You can request these charges be removed, but a discussion of the merits of cruise ship gratuities is WAY OT for this thread.
Alcohol is going to be an extra charge on either line. Drinks range from about $6 for the "special of the day" to about $9 for a call drink (and up from there if you're into premium booze), all plus the 15-18% gratuity. All food at the buffets and main dining room are included, but there may be "specialty outlets" like a premium restaurant or espresso stand that's an extra charge (plus, you guessed it, 15-18% gratuity).
You'll have many opportunities / offers to have your picture taken by the photo staff of the ship. There's no charge to have the picture taken, but prints can be expensive - around $25 for a single 8x10 (although individual picture prices drop the more you buy).
Most cruise lines are now including "fuel surcharge" language in their contracts. Typically it's worded that if the price of a barrel of oil is more that $XX when your cruise sails, they can add $xx per person to your account. This hasn't been invoked on our last few cruises, but with oil creeping back up toward $100/barrel, it may come into play again.
It looks like you'll be flying in from WI. The cruise line can book your airfare, but you can probably get a much better price on your own. The advantage to having the cruise line book the airfare is they will either hold the ship or make arrangements to have you meet the ship at the first port if your flight is late. The down side is you're stuck with whatever flight they give you, unless you want to pay even more for a personal preference schedule. So you might have a flight leaving WI at 6am on the day of the cruise.
I would strongly suggest flying in the day before and getting a cheap airport hotel for one night (Priceline's "name your own price" is a good tool for this). By flying in the night before you're less likely to have any problems getting to the ship before it sails if there are flight delays, you'll be less stressed by not traveling on departure day (even if you get in at 11pm the day before), and you'll probably still save money over the cruise line's airfare.
On inside versus outside cabins - you'll find the cabins are very small, especially on an older Carnival ship like Fantasy. You won't be able to do much more than sleep and shower in the cabin even if you wanted to. For a three day cruise, I'd say save the money and take the inside cabin. On a longer cruise we will occassionally take an outside cabin if there's a good deal, just for a little natural light for the longer trip.
Finally, just be aware that the really inexpensive 3-day cruises can be a "party barge", especially on Carnival. You're not going during spring break, so it may not be quite so bad, but if your trip is over a weekend you still stand a good chance of getting a large number of college kids that want to cut loose for the weekend (and Halloween weekend may be a draw). We vividly recall being berthed next to an older Carnival ship in Cozumel once, and watching two young men so drunk they could barely stand trying to help a third young man who couldn't stand at all, trying get back on the ship. (I don't know if there's a minimum drinking age in Mexico - maybe 12??)
Hope this helps, and bon voyage!
). BUt if you have 2yo, then Carnival lets non-potty trained 2yo's enjoy the kids club without a parent....
