Body Slam
It's not mentioned in the manual, but if you press jump and triangle at the same time, Obelix will do a body slam to the ground causing damage to everything but bosses.
Beat The Norse Hammer
To easily beat the Norse Hammer, pause the game and press Square. For some reason this allows you to end the boss fight.
Costume Change
Beat the game on any difficulty to select a different costume for Asterix.