Okay, let's fill the play area with rotating pickup objects. 
Before you start, let's get things organized. 
In the Hierarchy, create a new GameObject and call it pickup parent. 
In the Inspector, right-click the Transform component title and select Reset to reset pickup parent to origin. 
Then, in the Hierarchy, drag the pickup GameObject onto pickup parent. 
Next, make sure that you have the child pickup GameObject selected in Hierarchy, not the parent. 
Select the gizmo in the upper right of the scene view to switch to a top down view. 
Now use your scroll wheel to zoom out a little so you can see the entire play area. 
Okay, let's start placing pickup objects. 
Move the first pickup GameObject somewhere you like. 
Next, let's duplicate the GameObject. 
Make sure it's selected, and then go to Edit > Duplicate. 
You can use the hot keys control-D for Windows or command-D for Mac OS. 
Now place the second instance of the prefab. 
Using the hot keys, create some more pickup GameObjects and place them around the play area. 
About 12 is a good number for this game. 
To make this easier, you can move parallel to the ground or XZ plane by selecting the XZ plane at the center of the gizmo. 
Okay, that's 12. 
At the moment, the collectible objects are present in the game. 
But the player can't actually collect them. 
That functionality is the next thing you're going to add.