After you complete Banjo-Kazooie with all 100 jiggies then Mumbo Jumbo tells you of a sequel called Banjo-Tooie and shows you the secrets in the first game that can only be found when you buy Banjo-Tooie. These secrets will not be essential to completing Banjo-Tooie but when you've completed the sequal, you'll certainly want them! These secrets are:
This is found in Tresure Trove Cove. Somewhere along the line in Banjo-Tooie, you will acquire something which makes Sharkfood Island raise. Inside there is a large rock where you have to do a series of double jumps into many small caves until you reach the top where there is a large pink egg. The most likely probability is that you need to type a code in on the sandcastle floor to make Sharkfood Island rise. It could be something like POUNDTHETILETORAISETHEISLE (P.S. It is not this!!)
This is found in the door that won't open in Gobi's Valley where Gobi the camel gives you a honeycomb piece. After you've made the door open then you'll go inside and butt-stomp a switch which makes a sarcophagus open. Inside is a blue egg. This is a complete mystery!!! Or is it?
The biggest mystery of all is of course, the Ice Key. This is hidden in Wozza's Cave where the orange Jingo is. You will have to climb up on to the ledge and break an ice wall that is in front of you using some move that you get in Banjo-Tooie. Inside there is a large Ice Key. There actually is a way to get the ice key in Banjo-Kazooie by using a cheat code on a special piece of hardware called a Gameshark.
This is the first secret to be discovered by people outside Rareware. The X-Barrel can be found down in the cellar on the Mad Monster Mansion level. What happens is that you kill a ghost using your Wonderwing and then you can break all of the barrels using your beak barge except one barrel, the X-Barrel. By using the Door Opening code on a Gameshark it is possible to open the X-Barrel and reveal a door to a small room consisting of invisible walls. In the middle there is an invisible object that can only be touched.