Go to Banjo-Kazooie's Sandcastle in Treasure Trove Cove and start off by spelling CHEAT. Every letter you hit will make a "moo" sound to confirm that you did it correctly.

After you spelled CHEAT, you will be able to enter the following lengthy cheats to unlock the hidden stuff. The cheats are pretty much self-explanatory. Note that the letters won't make any noise.

To open the locked gate in the rock wall in Gobi's Valley, enter the following cheat. You will find the Blue Egg inside.

A DESERT DOOR OPENS WIDE ANCIENT SECRETS WAIT INSIDE