Our trip at the end of the month will be DD's 4th trip to Disney and she is 2 1/2 so I have a bit of experience with little ones at WDW. Make sure you bring a comfy stroller. It is essential. We always rent a City Mini, which my DD will nap in. She won't nap in her stroller at home. Use the baby care centers when you are nearby. I wouldn't make a special trip to go to them, but if you are close by I would use them for diaper changes, etc. They have a lot of changing tables whereas the bathrooms generally have only one, which can result in waits to change diapers.
We did not take mid-day breaks. DD napped in the stroller just fine. While she was napping we took turns riding things she couldn't. Take advantage of single rider lines for that. One day, DH and I rode EE 4 time each. We could have continued to ride it while DD was napping but decided to go get a snack. Some kids won't nap in strollers and if that is the case, you may have to go back to the room for a nap.
My DD LOVES characters, but some children don't so I would get a feel for that on the first day. They are super cute with the characters if they like them. The same with fireworks. DD loves fireworks now, but she hated them on our first trip when she was 9 months. My point is be prepared there will be some things the baby doesn't like.
Be prepared to let you little one get out of the stroller some. We set aside time for DD to play in the water areas and get some exercise out of the stroller.
Just kind of go with the flow with the baby. You probably won't get to see and do as much with the baby as you would without a baby, but it is a great age to take kids. Everything is new. My daughter's vocabulary increased so much during these trips. It was amazing.