Enter the following while playing:

Complete the game, get the full 100% in your stats menu and you will be greeted by a special cut-scene to watch.

You need to collect enough opals to buy these at the boomerang shop in Burramudgie. They're the red eggs.

You need to collect a amount of opals to upgrade/buy these boomerangs.

When you get hit, you can go swiim underwater come back up and have full health.

In case some of you don't know what the Fourbie is, it's the red truck that you use to go places. There is a trick to get opals while riding the Fourbie. It only works once per game. Not game as in the entire disc, but game as in a single file. The trick is to hold down the R2 button while driving on the road in the Fourbie and hold the the left analogue stick to the left or right. It doesn't matter which way you hold it though, as long as it makes the Fourbie turn in circles. After a few seconds of spinning, you'll get some opals. You can use opals to buy stuff. Enjoy this nifty little trick!