What was Jesus doing until he was 30?

Born in Bethlehem. 

Lived near Jerusalem until he was 2 yrs old when the Magi visited him.

He and his parents fled to Egypt until Herod died. That lasted anywhere from a few weeks to a few years. 

They returned, but not to Judea. Instead they moved to Galilee when Jesus was around 3 or 4.

The next biblical story takes place when he was 12 yrs old. He visited the temple and spent time discussing the Law with the leadership. Something uncommon for a child to do. They noticed something was different. 

From there we assume he worked in his family business. People called him Rabbi, which was a name usually reserved for  an official Rabbi who had gone through Rabbinical school, and become an older Rabbi's disciple... However, I don't think that Jesus did that. I think he worked in his family business and studied the torah on his own time. I think this, because people were surprised when he started his ministry. They thought of him as simply a carpenter (or the word could be translated Mason). They said things like, "where did this man get such learning?" (my paraphrase) if he were a true rabbi then the obvious answer would be, "from Rabbi Markowitz" or something like that. 

He doesn't show back up until the beginning of his ministry. 

There are apocryphal stories of Jesus healing birds etc. when he was a kid. Most scholars agree that they were written a few hundred years later by over zealous monks.

In biblical times, ink and paper (papyrus or velum) were expensive commodities. Written stories were short and to the point. They wrote from edge to edge with no margins to pack as much onto a page as possible. The fact that Jesus' childhood is a bit of a mystery is not all that surprising, considering that the early writers were not wealthy. They wrote what was important to the story. 

We are used to reading 100,000 word biographies in our modern low cost printing era. However, that was not so much the case in antiquity. So a missing childhood is not a big surprise.