I didn't figure this out until we had 2 kids, but kids/baby ear protection is SO worth it. WDW is LOUD (and we live in NYC). It is a must for shows, parades and fireworks, and many of the rides are surprisingly loud too. MK is especially loud. DD1 did a lot better with ear protection at shows and for fireworks, etc. If there's a chance your LO can get overstimulated, bring them along. My kid has no sensory issues whatsoever, but at age 4.5 would actually choose the earmuffs and enjoyed the shows, etc more. DD2, then an infant, slept through fireworks that way.
PPs have great advice here. Our DD was scared of the furry characters until about 2.5. Chef Mickey's was too much for her, but a great first character meal is Garden Grille in Epcot. Table service, and if you are on the outer ring, you really don't see any other tables or people except the characters. We started with DD sitting between us, and the characters would come and sit next to one of the adults (U-shaped booth) until she felt ready to come out to meet them. Chip was the one who finally broke the ice. (I see other people have mentioned this already! ha!)
Other advice: Get the Memory Maker! It will be hard enough to take pictures with a toddler, and this way you don't have to rush to get your camera out trying to catch a magic moment. Every picture will be a family picture. Be sure to ask for the "magic" shots - if your DD can pose, you will get some ridiculously cute pics with Tinkerbell, etc.
DD1 does love MK, but riding the monorail to Epcot is fun too. There is a surprising amount to do there with a little kid, and a stroller is a MUST. I didn't find HS worth the trek until DD1 caught Frozen fever, and even then it was a half day park for us. AK is lovely too, we try to go to Tusker House every time. The shows there are excellent (Finding Nemo and Lion King). We also spent LOTS of time looking at the animals. You could easily spend a full day (or more) there.
We are big fans of the midday break. *WE* needed a rest, and DD1 napped better at the hotel.
Also as others have said, let DD take the lead - on our 6th day or so, we made it as far as the entrance to main street, and DD played around and danced to music outside the Tinkerbell meet area, and then refused to go any further into MK. So we spent the rest of the day at the pool. Another day earlier in the week, DD was overtired/overstimulated, started to run off into the crowd as it was getting dark, and we went straight back to the resort. We had Wishes FP that night and had to miss them. On that same trip, we had a day where we rode the carousel 4x in a row, and Small world 3x.
Oh - not sure how your DD is about shopping and asking for things in stores. Seems like many rides end up in stores, so be prepared. We started teaching her about money at that age; she got pennies for various things at home, and brought a little coin purse. If she wanted to buy something, we told her it was "X coins" (and we'd pay the difference). If we wanted to be done with the shopping aspect, I'd be like, "how many coins do you have left? 10? ok, that costs 10 coins." (mean mommy). Some things that were too expensive, I'd just tell her it was 20 coins, or 30, and we couldn't afford it. It actually worked surprisingly well.
