We took our now 2 1/2 year old when she was 9 months old, and we'll be taking our newest in Septmeber, when he'll be 5 1/2 months old. With Mackenzie, I kept her in the sling most of the time, and she did just fine. Keeping a baby close helps prevent he/she becoming overwhelmed with the sights and sounds. We'll be wearing William in the sling too on our upcoming trip. Babies like to be close to their parents.
Make sure to bring a few wide-brimmed hats, some chemical free sunscreen (California Baby makes a GREAT sunscreen, its expensive but its better than baby being exposed to chemicals at a young age and it protects tender skin against burns), a blanket to drape over stroller when baby is in there sleeping, a few toys, plenty of diapers and wipes, a sippy cup if she's using one, and a bib. I forgot a bib when we took Mackenzie!
Backpacks are easier to deal with than a diaper bag.
Bring your own stroller. Disney rental strollers do not recline, and they are made of plastc. Wait, maybe the jogging-style strollers at AK are made of mesh or something...I can't remember.
Oh, one tradition we have on our children's first trip is to buy a Mickey shaped balloon from Mainstreet USA our very first day. You can't take them on the plane inflated, so we make a little slit at the botton on our last day, deflate it, fold it, and then when we get home, we hang it on the wall with poster putty. We save the string and weight, and hang that too. Its cute, and my 5.5 year old still has his on his wall!
Have a great trip!