Bomberman 64
Secret Battle Levels:

If you have the import version of Hudson's Bomberman 64,
you can find some cool hidden battle levels. You need the
Hudson JoyCard 64 controller for this to work. Go to the title
screen, put the "Slow Switch" on position Hu and wait five
seconds. You should now hear a sound that confirms that the
trick worked. Choose Battle Mode and you can access four new
secret stages!

Reported that you don't have to have the import version to do
the above trick. Just press the Start button continuously while
at the Adventure/Battle Select Screen.

Bombing for Gems (hint):

In blue resort 1, go up the stairs where you first start and take
a right and go up those stairs.Go a few feet straight and take
another right turn go up those stairs and bomb the drawbridge
down.Go over the bridge and go up the stairs ahead.When
you get to the top,there is a brick building in front of you.Go
to the side of the brick building with the windows.Look at the
windows.And you can see that only one of the windows is
open.Throw a bomb into the open window and wait a second.
The bomb will explode causing all the other windows to shatter.
As the windows shatter,guards and other enemies to fall out of
the windows.As a result, the enemies will die and drop gems.
Collect the gems and become rich.