Reference article
Complete Every Quest
Finish all 21 side quests after the main tournament story.
In-game objective
You're not done yet! There are still ducks out there that need your help
Check the quest list
Use 
Quests to find the 21 side quests. They are separate from these 24 main-objective entries.
Finish outstanding tasks
Revisit the quest givers and activity areas across the region. A quest counts when its reward or final conversation is completed, not merely when its items are collected.
Where this happens
This goal spans the whole region rather than one completion area.
Across the games
Compare games and editions on the 
Duck Life 8: Adventure page.