We took DD at 6 months and it was a lot easier than I planned for. Here is what we did, hopefully some of the suggestions work for you too!
1. Disposable everything!
We used the Playtex Drop-In system down there and it was a dream. We could toss out the used liner, rinse off the nipple, keep it in a ziplock until we could wash it. We also took a Munchkin bottle drying rack (Target ~ $7), a bottle brush, and dish soap in a travel container to keep in our hotel room. We worked off a rotation of 6 nipples (3 for in the park, 3 for the room and swapped them out on breaks). We took our formula AND cereal in those little 3 chamber thingies and again refilled at break.
Disposable spoons, bowls, bibs, and changing table covers were great. We would use them and toss them so we weren't hauling dirty stuff in our diaper bag.
2. We split our baby's time between the stroller and a baby backpack. Believe it or not, she got sick of the stroller after so long.
3. Fireworks didn't go over well. She didn't like the sound.
4. We took our own crib sheet to use in the borrowed pack n play. It was an old one, we just left it behind when we came home.
5. Full brimmed hat, sunscreen (for the face)
6. Take your time. If your LO is getting overwhelmed, duck into a longer attraction for some (possible) A/C and a good rest. The train, riverboat, HoP, CoP, Tiki Room, etc are all great for this.
7. DD sat and watched Monster's Inc Laugh Floor and was actually part of the show.
8. Know where your baby care centers are in each park.
That's all I can think of right now. I agonized before our trip - I was so nervous of how she'd handle the parks and the characters and she did a lot better than I planned for. All of the characters were great with her. They were very gentle and moved quietly around her. I have a great series of pictures with her and Mickey at Ohana! Goofy at AK came over to her (I didn't take her out of the stroller at the character spots b/c I was afraid she'd get scared) but Goofy came over to her stroller, croutched down to her level and waved to her. I thought it was pretty cool.