In its simplest form you want to be looking at using a flag variable like "Carrying" to see if you're carrying something, and use a private variable as a flag for instances of your objects you want to carry.
To pick up an object:
+Carrying is 0 (false)
To drop an object:
+Carrying is 1 (true)
---[Update object with new position coords]
And use an Always to position object above player:
---[Position object above player]
You can improve it by making sure that you can't pick up whilst overlapping more than one object, and you can't drop whilst overlapping another object.