Remember that your child won't miss what they don't know about. Just because you couldn't score that Mickey Bfast meal doesn't mean the kid will be disappointed and broken up about it, they won't even know it was an option. So they will have as much fun playing at a jumping water feature in EPCOT as they would being IN a parade or whatever.
Take breaks! Probably the biggest issue I've seen with 2 year olds is overtaxing them into a meltdown. If the heat is an issue, the stress mounts, fatigue sets in, etc, before you know it everyone is yelling at each other. So a few hours in a park followed by a nap, then some pool time, then a low stimulus meal might work better than an all day park blitz, especially for a long stay. Not forcing them into an experience YOU think will be fun is another one. If they are scared of an attraction/character (my 2 year old gets scared of some odd things) I wouldn't push it no matter how sure you are that they will love it in the end. Do something else and try again another time.
Be flexible. Disney allows for a lot of pre-planning (some would say it REQUIRES it) but you need to be accomodating of the family mood. Have that reservation cancellation number on speed dial in case you need it. Talk to customer service if you need help getting fastpasses or reservations moved around because the kid was sick for a day. That is what they are there for (fielding complaints isn't really their prime task).
If you are bringing a stroller then read up on some of the threads here about stroller security. DO NOT leave valuable things in your stroller, DO mark it in some obvious way so you are sure you have the same one (and so OTHERS can be sure this isn't their stroller!).
EDIT: Some restaurant recs. The Seas in Epcot is a sit down place with big aquariums. My kid loves that stuff. T-rex cafe in Downtown Disney, Rainforest Cafe in AK. Biergarten buffet in Germany has a music show and allows kids to come up and dance. Sci-fi cafe in HS shows old 50's sci-fi movie trailers. Pretty hokey, may or may not scare your kid or make them really happy. The 50's cafe place in HS is a pretty good sit-down place for kids as well. Also, it may go without saying, but you CAN bring in your own food and drink in a small cooler, just no glass/alcohol. Water can also be gotten for free if you ask.