Instructions
Begin a game file on "Banjo Kazooie: Nuts &Bolts." This is required to activate "Stop'n'Swap" and get the "Ice Key." If you already have save data for "Nuts &Bolts," you will be able to get the key.
Begin or continue your file in the "Banjo-Kazooie" game. It must be the Xbox version, as the Nintendo 64 had problems that did not allow the "Stop'n'Swap" quest to function.
Play through the entire game until you reach the Freezeezy Peak level.
Go into Wazza's cave on the mountainside. Climb the ledge inside and grab the key. Kazooie will comment that you should save the key for later.
Return to your save file in "Nuts &Bolts." You will see a short cutscene of your key being added to Showdown Town. Go to the location it appears and collect it. Collecting the key will unlock various effects, depending on whether you used it in the Xbox version of "Banjo Tooie" or went straight to "Nuts &Bolts."