Not sure how old your baby is, but I bring:
A sippy cup for water
Bottles and formula
A clip on fan (I just got a new one off
Amazon if you need a recommendation)
Sunscreen
Change of clothes
Burp cloth
Diapers & wipes
Snacks
A small toy or two
A hat
Antibacterial wipes
A binky
I also have a soft "anywhere high chair" thing that can be put on any chair to make a high chair, just in case I need it!
One other thing - make sure you take advantage of the Baby Care Centers in each park. They are a lifesaver. I spent more time in the Baby Center than I did in the parks in May, because DS was hot and crabby and we needed to enjoy the A/C! They have nice changing tables, high chairs, Disney movies playing, and all but the MK have room to play and have some freedom for older babies. They also have private nursing rooms, rocking chairs and a microwave. And they sell any and all baby gear, so whatever you may have forgotten they have! I know they have Gerber formula, not sure about the other kinds.