Once you have completed a level in Story mode, as covered in the Walkthrough section, you unlock that level for Free Play mode. In Free Play, you can go through the level again. However, this time you can use any of the characters you have unlocked—not just the story characters. This allows you to get all the collectibles for that level.
Each level contains 10 minikits and 1 Stan Lee in Peril. Some of these could be collected during Story mode. However, a vast majority are only obtainable during Free Play mode. This section provides the details on how to get all 150 minikits and 15 Stan Lee in Perils for the 15 levels of the game. For information on solving the puzzles to complete the level, refer to the Walkthrough section which takes you step-by-step through each of the levels.