Hidden volleyball game:

This is available only in two player mode. When in the Cove 
(Sauron's stage), either player must perform a combo by completing 
two quick attacks in succession. Two humans will appear and bow. 
The player closest to the human must knock the human into the air. 
The other player must hit the human back toward his opponent 
before it hits the ground. The first player should continue to hit 
the human back to his opponent. After a while, a net and judge 
will appear, allowing volleyball to be played with humans. 

Hidden bowling game:

This is available only in two player mode, on any stage. Both 
players must play as Armadon and perform the Spinning Death (1 + 3, 
Away, Toward, Down) move simultaneously three times in a row. If 
done correctly, both players and their worshippers will walk off 
screen for a bowling game. Humans are used as pins, and the players 
bowl themselves by using the Spinning Death move. 

Skydiving cows:

This is available only in two player mode. When in the Ruins 
(Chaos' stage), one player must be playing as Chaos in a Sudden 
Death round. Allow the match timer run down to about two to three 
seconds. At the this point, the player using Chaos must perform 
the Fart of Fury move (2 + 3, Toward, Up, Away) so that the cloud 
remains in the air after time expires. If done correctly, the 
falling meteors will turn into falling cows. 

Hint: Diablo's Pulverizer move:

Hold 1 + 2 then press Up, Down.