Things I used the most during trip with 6-month-old:
1) Purell
2) Light, big blankie (for nursing)
3) Stroller (never used the carrier), had family to help hold DS, and the stroller came in handy naps and piling everyone's extra stuff in while on rides
4) Two extra sets of clothes in diaper bag (one "warm" set, one "cool" set...we were there in December, and the days started off cool but got really, really warm)
5) Sun hat
Disney is very nursing friendly, and no one really pays attention to you if you choose to nurse in public.
I wasn't sure about nursing DS in the parks (bought my pump, cooler, bottles, etc.), but ended up doing nothing but that. It was so easy! By the end of the trip, I'd nursed DS on out-of-the-way benches, in shows, on rides, standing in line for rides, just about everywhere, always with our blankie tossed over my shoulder. No one, and I mean, no one gave us a second glance, and I was there during a very busy time of the year. I had to actually look for other nursing moms...and they were everywhere! But I was so busy having fun that I never noticed there were so many other nursing mothers, even though I was one of them. Everyone, including some members of my family who are past nursers, usually just thought DS was sleeping when they saw us with the blankie. It was very convenient for DS, and I had a very content baby for the trip.
The stroller was great for DS during naps, and for holding other peoples' stuff (we were there with a large party). Alwasy found my stroller after rides. You should be aware that the strollers are sometimes moved by CMs for traffic flow reasons, but it was usually easy to find the new "parking lot" and our stroller...you may want to tie something visible to your stroller to locate it easily.
Honestly, it was easy taking a 6-month-old to the parks. We took DS on the calm rides, and took turns on the wilder rides. He was very content to nap in his stroller at that point, and he didn't want to get out and walk yet! He was easisly entertained during adult meals. And taking him along forced DH and I to go at a slower pace, which we really enjoyed.