One thing that can happen is that you can end up running around on the ocean floor. Even though you are underwater, the game acts as though you are on dry land. (It was possible to do something nearly identical to that in the Click-Clock Woods level of Banjo-Kazooie through use of glitch.)
If you get yourself swimming, you can swim through the entire island and look around to see lots of polygons floating in mid-air!
Look up in first-person view to see all kinds of weird stuff. This also bears a strange resemblance to the Click-Clock Woods glitch of Banjo-Kazooie that allowed you to see through the giant central tree.
That reminds me of the floating palm tree in Aidyn Chronicles.
There's a beaver above you!!
Chunky looks like he's floating in zero gravity!
This glitch is so huge and so easy to do, I'm really surprised Rare didn't catch it before the game's release. It works just about anyplace there's water, but I didn't want to mess up my game so I never experimented with it much anywhere but DK Isles.
And there is something very, very interesting hidden under DK Isles...
If you look up and see something that looks really weird and out-of-place like this, you've found it (go to the next section of the Donkey Kong 64 Bananza to read more...)