Each playset will have an area or objects that you can customize. Here's all the ones i know so far (note that these are really short descriptions)
Pirates of the Caribbean
What you can customize: Your ship and crew.
Monsters University
What you can customize: your campus and fraternity row.
The Incredibles
What you can customize: your super hero lair
Cars
What you can customize: The buildings and where they are placed
The Lone Ranger
What you can customize: The train that moves around the world and the townspeople
I'll post more when I get the information.
EDIT: I changed some of the information in the post based off of Pizza Planet's description.