You get 100% when you find all collectibles.
You get gold bricks from:
Completing story areas
Getting the fastest time in races
Completing story locations
Finding them scattered throughout the island
Tornado of Creation builds
Defeating Dojo masters
You get Ninjanuity tokens from completing heroic milestones during the story.
You get an ancient scroll for getting every Ninjanuity token in the game and another one for achieving Gold status on every dojo. The rest are scattered throughout the island.
There are 17 character tokens you get for playing the story. The rest are scattered throughout the island.
You can also use Gold bricks to improve the island.
Here are the videos that show where everything is:
Full credit goes to Packattack for his awesome videos.