Cheat menu:

Press Z at the title screen to display the cheat menu. Accumulate 2000 
frags total with all the characters to unlock all the cheats in the 
menu. 

To unlock all the options quickly, build a saved game with a character 
that has over 500 frags. Copy that character into the three other slots 
on the controller pak. Then, load all four of these cloned players to 
the game. All cheats will now be available in the menu since the 
combined frag count will be over 1000.

Hint: Exploding napalm:

Hold B when you using the napalm launcher to make all its shots explode.

Hint: Getting Medals:

You can get medals in multi-player mode by simply by setting a frag 
limit and winning that match. Also, you get four medals in single 
player trials by defeating Bastille, Syra, Simbiont and Tal Set.

Hint: Medals:

There are fifty medals in the game.

Hint: Extra multi-player characters:

Successfully complete the single player trial mode as Turok to unlock 
another character in single player trial mode. Each time that this is 
done, another character will also be unlocked in the multi-player mode. 

Hint: Invincible monkey:

Unlock and enable the "Infinite Ammo" option on the cheat menu. Set the 
other game options as desired, but you must have the minigun as a weapon. 
Play a game of Frag Tag (kill the monkey). Select the minigun and hold 
down Alternate Fire to activate the shield. Since you have unlimited 
ammunition, you will not run out. When you are selected as the monkey, 
you will keep the shield as long as Alternate Fire is not released.