My boys have loved Epcot since they were little. We have always had that be a two park day for us, there really is so much to do if you take your time. The passports in World Showcase are so cute. I made them myself using designs from the creative DISigns section. The kids love getting their passports filled out with messages in another language. They always loved bringing them to school when we got back to show their classes.
We also spend a lot of time in the Seas building after riding Nemo. There is so much to do in there. They love Turtle Talk, playing by the Bruce statue, watching the animals, and the interactive computer game about sharks. After the Figment ride there are a bunch of cute things for them to play with also. There are a few things to do after Mission Space also. Even if you don't ride it, you can go in through the gift shop and go by the playground and computer games. We used to love the Great Piggy Bank Adventure too, but I think that closed.
In World Showcase we love the Agent P game. We play it multiple times each trip. It is really cute! We also like doing the passports, getting snacks from different countries, the Kidcot Fun Stops, seeing different characters, and watching then entertainment in the various countries. Their favorite is watching Sergio in Italy. There is so much to do there!