From where you got the Pink Egg, use the flight pad and fly over to where the waterfalls are (to the west of Grunty's old lair) and go through the opening on top of the waterfall, then swim/jump upstream until you're in the next room. You'll find another waterfall here with a ledge on top of it. Grip Grab along the ledge and climb up directly behind the waterfall to find a hidden tunnel. Go through it to find another Banjo-Kazooie game pak inside the cave. Break it open to get the Blue Egg.