1 – For discovering all 20 objects; considering that with each run only 10 out of 20 possible objects are generated, you will need to repeat a level until you reach 100% score OR use the cheat code 777 to show all objects.
2 – For completing the bonus stage, which is unlocked only after you reach 100% score in the previous stage; now all 20 items will be displayed in the scene at once.
You can trigger this mode by hovering on the 100% score until the title of the level changes (the border changes color too).
The screenshots below display the locations of all 20 objects inside a level.
Additionally there are two achievements that can be obtained in any level / mode: