Normally, only Tiny can get through this door to the Banana Fairy's room, and then only by using the shrinking barrel on the right there. But using this trick, even a fully-grown Lanky (or any other Kong for that matter) can get inside.
Swim just off the coast of the island, press Z once to dive, then rapidly tap the B button. You should go right through the ground. Try to swim inside the island and press R and C-Up to reorient your camera once inside. Look for that trapezoidal formation seen right in front of Lanky in my screencap. That's the entrance to the Banana Fairy's room...
In first-person view, this is what it looks like.
Jump up into it, and then run forward.
Although you're actually going through it the opposite way you would normally go with Tiny, you'll still trip the gate when you hit the right spot.
Tah-dah! You're in the Banana Fairy's room! Not much to do in here but collect some crystal coconuts. Unlike entering, you don't need a trick to leave. Your Kong is scaled down enough to leave through the exit no matter who you bring in here (even Chunky).
Um...sorry. Couldn't get the Banana Fairy's head in this screen capture because she acts like such a freak.