Our dd was 15 mo when we took her. She is now 15...sigh...
Anything that I need to make sure I pack?
Diapers, wipes, disposable bibs (do they still make them??), Clorox wipes, sippy cups, ice packs, zip lock bags for left over food...we don't do dining and I would buy my dd a kids meal and she would have it for leftovers. I know how horrible but I have leftovers at home all the time!
Laundry detergent and less clothes. We wear 2 outfits per day, there are 5 of us and our last trip was 11 days...that would be a lot of clothes and suitcases.
you packed but didn't end up needing?
Toys! I brought a bunch of small toys for her to keep busy with in the room while dh and I got ready. She never touched them! In the am while dh and I took turns showering she was ever still asleep, getting dressed, sun screened or eating breakfast as we bring food to eat in our room.
Best places to eat/snack/rest
Eat-I would look at menus. It's all subjective. We live chef mickeys and whispering canyon cafe and Trex and Wolfgang pucks express. We don't do a lot of sit downs because we don't do dining and can't be tired down to eating at a certain place at a certain time, our max for a sit down is 2 per trip.
Snack-the same thing, although I bring snacks for my kids to the park as well.
Rest-no place better then the room. We all nap these days! We are out the door by 7-7:15 depending onwhich park and which way we are getting there-monorail, boat or bus. We return just after we ate lunch and nap until 4 and then shower and return to the park until 10-11. Yes even my then 15 month old did that with out issues. The same with my then 22 month old and lastly our 2 1/2 yo (the last trip they were 12, 9 and 7 and yes we napped then too). But we do plan a non park day after ever 2 park days just to recoup...well they recoup, I am still up at 6, in my bathing suit and I start laundry!
Best entertainment for the journey down there?
For that age, whatever she likes. Movies can only hold interest for so long. Color wonder? Things that aren't messy I would say.
What do I need to make sure I bring to the parks?
A smile and patience. After all it isn't about you anymore there is a kid. I had plans to do reader swap with my dh for space mountain because I rode that every time. Well that first trip with a kid, I didn't care. It was all about seeing her have fun. Then we went again with our 2 kids and my parents came for a bit so I had planned to have them watch them while dh and I rode...well I was pregnant with our 3rd so that didn't happen!
We go with the flow, we know which park we are going to in the am, as long as there are early entry hours we go to that park, have lunch where ever we end up and at night we go where ever we want dinner...either MK or Epcot.
Other things..a change of clothes if you have a messy kid, zip lock bag of wipes, table toppers, diapers, sow,thing to put on the changing table (I used the huge supply of small blankets we had), drinks, small non messy snacks, hats, sun screen. Heck when our son was 22 mo, I gave him ice cubes to play with in the stroller. It was hot, he was hot, he loved water and ice so it was a win win!
Rain ponchos and something to cover the stroller in case it does rain.
Anything your child did/didn't like at that age?
Each of mine were different. 15 mo-hated the face characters, would not go near them but pooh, Mickey, etc...she was all over them. 22 mo old, loved them all but as long as we were holding him and he was clinger to us for dear life, 2 1/2 yo did what her siblings did, had no fear.
I know they all got a little nervous in the dark rides, any of them-space ship earth, POTC, HM, etc....
Our oldest didn't like to sit. She was and still is a skinny little thing and had a ton of energy but she was not one to run from us (non pe of our kids did). So the first day in the MK after we took her out of the stroller to ride a ride she did not want to sit back in it. She then realized that we were going to another ride and was fine after that but we did let her walk as much as possible.
General advice for the trip
Have fun and go with the flow. Do make too many plans because things will change, people get tired, someone might not be hungry at 5:30 for dinner because they just had a snack at 4:15.
Golfing? 
