You can do a lot with an 18 month old. We took Michaela to
Disneyland when she was 18 months old. You have to set your expectation level. My wife and I got to ride very few big rides together. But we did all the dark rides, shows, etc. She
LOVED "It's a small world"
Word of warning... keep a tight grip on your little one while riding the Peter Pan's Flying Boats. Michaela got so interested in the miniatures that she just about jumped out of the boat.
Suggestions
All of Fantasyland except the teacups.
Dumbo first, ONLY if your little one is that kind of kid.
countyr bear
tiki room
buzz lightyear
TTA (depends on kid)
toontown obviously

The train
Tom Sawyer
Pirates of the Caribbean
Jungle Cruise (a must)
Magic Carpet only if they are not jumpers
Too young to be scared in HM so I'd do that too
Another word of warning. Some little ones are not "brave" yet, so character pictures may need to include another family member, but get them anyway. Anna (my 11 yr old) was VERY timid. we couldn't do anything like that. Michaela (now 7) loves to see pictures of herself and the characters when she was in DL. Stephen and Kaiti (my oldest two) on the other hand marched right up and got pictures with Mickey, Minnie etc when they were that age.