The father of Sophie's child is actually not Sky, but rather an actual Greek god.
Most likely Zeus, given his reputation. Which means that, by the end of the movie, Sophie has essentially given birth to a demigod.Bonus: Sophie knows this, but omitted this information from Sky (either that or she straight up lied to his face) because they were going through a rough patch and she wanted to make amends with him. Telling him the full truth would've just made things worse.
Donna met with Harry again on the island.
This would explain how he got her the guitar and why he is mentioned third in the diary entries from the first film. It would make sense for him to remember their last summer more fondly, and Donna did write that "he showed up out of blue" and it sounded vaguely like she'd known him previously. And also, poor Harry most definitely deserved more than what he was shown in their little one-night stand.