Hey, Kathy! Thanks for the question. Our littlest guy (who from henceforth shall be referred to as "The Biscuit" in this answer) was 18 months last time we were in Walt Disney World. He had the time of your life (can't believe I just succumbed to a Miley reference.)
With your little girl being as young as The Biscuit, they are still so impressionable. It's important to ease them into attractions so they can become accustomed to the experiences.
If your little girl is apprehensive about rides, start out very slowly with the Carousel in Fantasyland. The entire experience is plain to see going on to it. To an 18 mo. tot, it's more like a souped up merry go round. its a small world is a good next step, as it exposes her to the concept of the dark boat ride in an uber merry setting. While normally I'd recommend riding on Dumbo first to minimize your wait, in your case, it may be a good third ride to expose her to a gentle flying experience. If you think she may like it, Google "chuck bubba."
If these three attractions pan out, you open the door for more, such as Peter Pan's Flight, The Many Adventures of Winnie the Pooh, Magic Carpets of Aladdin, the teacups. The Biscuit liked shooting things in Buzz Lightyear. He also has an affinity for the Jungle Cruise and TTA. See how your daughter is doing at the end of the day for attractions like Haunted Mansion and Pirates.
In terms of character experiences, she may like seeing characters from afar. The Dream Along with Mickey show (multiple performances each day on the castle forecourt) will have the Fab 5 along with princesses, their fellas and my favorites: Peter Pan, Hook and Maleficent. Same concept plays out for parades, where you can also find Pooh and pals. Also hit the face character greetings - they're much less daunting for a tot.
If you have any other questions, come on back and ask another!