1) New Camera Views - From the Control Options where you set the keys, the last hotkey is blank. Set that hotkey and press it 5 times. That will activate the New Camera Views secret. Just press F1 to scroll through the different views. Press the hotkey again if you want to turn the new views off.
2) Exploding animation - Reload with the PP7, then shoot it 6 times. Press F3 (IR Goggles) to make your player do a strange exploding animation.
3) Create Stormtroopers - See the Instructions Booklet to read about this feature.
There are also several secrets in the GoldenEye Bunker level.