Advice? Umm, the best I can give would be "relax"! Judging by the tone of your post I wonder if you and I are alike. We took DS last May when he was 17mo and I was a nervous wreck over the whole notion of taking him on ANY vacation, let alone WDW. I nearly had an anxiety attack just before we entered the airport for our 3.5 hour flight to Florida. I was extremely worried about how DS was going to make it through that flight (we had never even taken a road trip any longer than an hour). I also worried/freaked about how he (I) was going to handle the rest of the trip - what if I forgot something important at home (turned out I did), what if he is a total toddler monster and has meltdown after meltdown, what if he won't sleep good (not a good sleeper to begin with), the list went on and on!
Well, let me tell you - I soon learned that I had (practically) NOTHING to worry about

Honestly, DS was nearly an angel during the trip! Oh sure, he had a few minor (and I do mean minor) meltdowns but you know what? He has far more at home than he did the whole week were in WDW on that vacation. I would definitely do that trip over and over again if I could. DS was absolutely adorable with the characters - LOVED them and they seemed to love him right back

I'll never forget the look on his face and how he reacted when he first laid eyes on BIG Pooh
Alright, now you did ask for some advice - here's a little of what I found most useful/helpful for our trip:
FLYING - First and foremost, if you are flying - DON'T do it at night!!! I can't stress this enough! We had a night flight on our way home and it was horrible - DS and another 20mo tot (who sat right behind us) did nothing but take turns screaming the ENTIRE 3.5 hour flight home - seriously

On the other hand, the flight down, a 6:30am flight was PERFECT! We woke DS up around 4am and despite the fact that he didn't fall back asleep until we reached AKL around noon (told ya he wasn't a good sleeper).
Another thing I wouldn't do any differently - taking the carseat for the flight. Yes, we could have saved $$ and DH's back (from carrying the carseat) if we just let DS sit on our laps but I was too paranoid that DS wouldn't actually sit on our laps for more than 5 min (he has NEVER been a kid who sits unless he's strapped in). By taking the carseat I figured that DS associates it with having to sit for longer periods of time. Don't know if it was my theory proving correct or not but for whatever reason DS did sit (and not complain about it) for the whole flight down.
OTHER ADVICE - We went back to our resort each day in the afternoon for DS to nap. We also tried to stay as close as possible to his regular bedtime. There was only one night we kept him out later (9ish). Really, our whole pace was set by DS.
We used tottoos every day for our own peace of mind.
http://http://www.tottoos.org/ Got many compliments on them and I will definitely use them for future outings.
We packed diapers for the trip. We did use almost all of them and then we ended up with much needed room in our luggage for souveniers.
I also brought plenty of snacks/finger foods for DS to munch on throughout the days (dried fruit, cereal, cereal bars, crackers, etc).
We did Crystal Palace and Chef Mickey's while there and DS LOVED the characters! For us it meant being able to see DS interact with characters without having to wait in long autograph lines - a definite must do IMHO.
We took DS on almost all the rides possible at MK, AK, and EPCOT (we skipped MGM). He loved everything - especially Dumbo of course! Actually, he loved them all, he even cried because he didn't want to get off Spaceship Earth
There are 2 things I would have done differently - 1) I would have paid the extra $$ (if I had it) to stay at one of the monorail resorts. The time it took to take a bus back and forth from the parks and AKL was really time consuming. We did it though because having a regular nap each day was a must for DS (not a good sleeper and has never slept in a stroller - even as an infant).
2) I know it's probably a no-brainer but I had forgotten to pack DS more than one pair of shoes. Turns out, DS didn't want to ride in the stroller AT ALL and ended up walking for at least 85% of the whole trip (no kidding). By the 2nd day the poor little guy had little red marks on his ankles from his sandals and we ended up buying him a pair of
crocs (which he LOVED). I just wish I had planned better in that respect.
Oh, and of course I wouldn't have taken a night flight home
I hope at least a little piece of this is helpful to you. Just go and have fun - that's what it's all about!
