Acts 1:3 has the following tidbit.
I am wondering what Jesus may have taught during these forty days that was not recorded elsewhere. Are there any canonical or non-canonical explanations of this period?
There is NO information canonical or non about those 40 days. All we know is that he showed Himself and spoke of the Kingdom.
A really good question might be, why 40 days?
It symbolizes a period of testing, trial or probation. It can also mean or symbolize a generation of man.
God flooded the earth by having it rain for forty days and nights
Moses' 1st 49 years of life shows him from growing up in Pharaoh's household and living in Egypt.
The 2nd 40 years of his life is spent as a shepherd in the wilderness until God calls him at 80.
The third 40 years of his life he leads the Israelites out of Egypt and wanders the wilderness until he dies at age 120.
Moses was also on Mount Sinai, on two separate occasions, for 40 days and nights (Exodus 24:18, 34:1)
He also sent spies, for forty days, to investigate the land God promised the Israelites as an inheritance.
He wandered 40 years in the desert.
The book of Exodus, in the King James translation, is the only book that has 40 chapters.
The Judges who served 40 years include Othniel, Deborah and Barak, Eli and Gideon.
The first three human kings over the children of Israel, Saul, David and Solomon, each ruled for forty years.
Abraham pleaded to save Sodom & Gomoarh if only 40 righteous people were found.
Both Isaac and Esau were forty years old when they were first married.
Elijah went 40 days without food or water at Mount Horeb.
Jesus was tempted by the devil many times during the 40 complete days.
The prophet Jonah powerfully warned ancient Nineveh, for forty days.
Ezekiel was told to lay on his right side for 40 days for the Kingdom of Judah.
Forty years after his crucifixion the Romans destroyed the city and burned its beloved temple.
The Bible was written by 40 different men.