Hi Amy,
Don't worry - you are going to have a wonderful trip and planning ahead is a great idea. You can enjoy any of the rides where your 18 month old meets the height requirements. Sounds like you've already visited the
Traveling with Little Ones section on the Disney World website which includes a PDF file listing the attractions with height requirements on the right side of that page.
If there are some attractions you'd like to do where your little one does not meet the height requirements, you can take advantage of the
Rider Switch program.
Having said all of that, there are some rides without height requirements that may frighten small children (i.e., Haunted Mansion & Snow White's Scary Adventure). I'd also suggest sitting in the rear of the train as the whistle may scare small kids. Even the loud booms from the fireworks can be a little much for the little ones so you may want to cover your wee one's ears.
I also suggest introducing the characters slowly. If your 18 month old seems fine with them from a distance, then proceed but keep in mind that the closer you get the bigger that mouse can get and you don't want a traumatic first experience. Of course, every child is different but it seems that most kids that age do pretty good with the characters but don't be surprised if you come back around the age of 3 and that changes dramatically.
Wishing you many magical memories :-) Allison