From the early days of the Church, believers painted scenes of the birth of Christ beginning in the catecombs. These scenes became a staple of Christian life and carried on through the years. In the time of St. Francis of Assisi, the images had taken a new form: faithful Catholics would set out mangers in front of their local church, but these mangers were often jeweled and made of gold to represent who they held. St. Francis marvelled at this and felt that the people were forgetting the humble, poor birth of our Lord Jesus.
So in 1223, St. Francis created the first living Creche in a small cave in Italy. On Christmas eve he gathered with others to act out the Nativity scene in all it's impoverished glory. This re-enactment is credited as the first Creche (which is French for "manger") and from that point forward the creche was designed in a more realistic way by depicting a stable scene.
A creche traditionally includes two animals: an ox and a donkey. These were the two animals participating in St. Francis' creche because he wanted to allude to Isaiah 1:3 which states "The ox knows its owner, and the ass its master's crib; but Israel does not know, my people does not understand."
Another important consideration is that the shepherds and the wise men shouldn't be at the creche together. These days, we tend to simply sit all the players out and leave them there for the season. However in many countries they add and remove characters based on the day. So for example, the manger should be empty until Christmas day. The next day shepherds show up. But not until early January do the wise men show up (by which time the shepherds have returned to their duties). And the nativity should stay up until February 2nd, which is the feast day of the Presentation of the Lord.
And your thought whacking out the nativity scene was the easy part of Christmas decorating!!!~