There are 56 character quests scattered throughout Chronopolis. Completion of each quest awards you with a character card.
Every area of Chronopolis except Knowhere and Kang’s Citadel is accessible after completing Level 5, ‘Hydra Hijinks’. Those two are accessible at the end of the story. Each area in the hub has icons representing activities to complete or items to collect. The quest icon is a dialogue bubble. Make your way to the icon to talk to the quest giver. Note that some quests are not accessible until you’ve completed other quests or races. Also note that in order to complete some quests, certain character abilities may be required.
The quests involve you going around various areas fetching items, defeating enemies or solving puzzles, to name a few. Each quest only takes a few minutes to complete.
The achievement will unlock when you’ve completed all 56 quests.
For a 100% text walkthrough to all the story levels, Gwenpool missions and Chronopolis, refer to this list by GameFaqs:
https://gamefaqs.gamespot.com/xboxone/209993-lego-marvel-sup...For a 100% video walkthrough to all the story levels, Gwenpool missions and Chronopolis, refer to this plsylist by packattack04082:
https://www.youtube.com/playlist?list=PLYpDU5ElRBfmlKGMF80dW...Edit: According to Schinderdiv, you may have to do the "Helicarrier Clean-Up" challenge again to get the final side quest to unlock.