Your 20-month-old should be able to ride any ride that does not have height restrictions, etc.
When DS took his first trip at 15-months he rode...
MK: all the Fantasyland rides, Buzz, TTA, Magic Carpets, Jungle Cruise, POTC. Essentially everything except the mountains and Barnstormer.
EPCOT: Spaceship Earth, Figment, Living with the Land, Ellen's Energy Adventure, Maelstrom, the Mexican water ride.
AK: the Safari, Triceratops spin
MGM Studios: The Great Movie Ride
Those are the only ones I can think of off the top of my head.