An 18 month old can actually ride a lot of things such as:
MK: Small world, jungle cruise, train, country bear jamboree, Buzz, enchanted tiki room, peter pan, transit authority, pooh - the list goes on really. If the child needs a moment to run, the pooh park across from pooh ride is good. If the child needs a nap and doesn't fall asleep in the stroller, Hall of Presidents could do it for you. Not to mention that a lot of these rides are places to keep cool.
Epcot: we did spaceship earth a lot, because there is no line and its always cool. The Imagination ride is okay, but they blow air on you at one point and that can be scary. Nemo ride is good (just one scary fish part). Can get out of the stroller in living seas to look around. Living with the Land is good for 18months. Mexico ride is good. There are water spouts in the walkway in-between future and world showcase. I always threw a few swim diapers and extra t shirts or onesies in bag for water thrills. An 18 months old sometimes likes to see things going on at world showcase such as the drums in Japan, trains running in Germany or playing in the maze at England.
Don't really recommend AK for 18 months (But thats just me).
MGM: Playhouse Disney good. Also, MGM is good for meeting Characters.
Character dining is always good. Chef Mickeys, Liberty Tree Tavern, Garden Grill Cafe
Each park has a baby station bathroom. I can't remember what they call them but find it on the map and go there for cooling off changing diapers etc. My main advice though is to stop and smell the roses, don't pound em too hard. Make sure you get the nap in and when you do go have margaritas in Mexico.
