There is a point where it is completely dark and then you hear the sound of a train coming by you. There is an imagination playground when you come out of the ride that is fun. There's an area of the floor that makes different noises and an electronic "orchestra" you can conduct with your hands.
6 month and 16 month olds might not be as afraid of some rides as they will be when they get a little older. Kids that little are often fine as long as they are with mom and dad. When they get a little older they start to think about what is happening (so a 16 month old may think Haunted Mansion is great, but once they get to 3 yrs old, they are not so sure about those ghosts).
When my kids were little, they liked Living Seas (all those fish and the water are very relaxing). It's even better for kids now because it's themed to Finding Nemo.
Another ride that's nice is the one in Mexico. It's sort of a Mexican version of Small World.
Rides in Epcot that are dark include Norway (also has a fall, trolls and a giant polar bear), Honey I Shrunk the Audience (dark and has 4D effects, like mice and snakes), Spaceship Earth (not scary, just dark and maybe long for a 6 month old), Universe of Energy (dinosaurs, some loud noises and some complete darkness). If it's open, Cranium Command has some loud noises, not really scary, but startling.
Most of the other things at Epcot are either sort of long for little kids (the France movie is very nice with beautiful music) or they have a height requirement.