Reflect on Eccl 3:1-17.  Spiritually speaking, what do each of these mean?  For example, spiritually, what does a time to be born and a time to die mean?

Eccl 3:11
He has made everything beautiful in its time. He has also set eternity in the hearts of men; yet they cannot fathom what God has done from beginning to end.

Rom 1:20-21
For since the creation of the world God's invisible qualities-- his eternal power and divine nature-- have been clearly seen, being understood from what has been made, so that men are without excuse.  For although they knew God, they neither glorified him as God nor gave thanks to him, but their thinking became futile and their foolish hearts were darkened.

God has “set eternity in the hearts of men” so that “men are without excuse”. There will be no excuses at the judgment.

Eccl 3:14
I know that everything God does will endure forever; nothing can be added to it and nothing taken from it. God does it so that men will revere him.
Nothing added.  Nothing taken away. A scientific principle is that matter and energy can neither be created nor destroyed.  Energy changes to mass and mass to energy.  Pure science points to the Creator.  As an example, consider the Big Bang theory.  Spiritually speaking, the unapproachable light of God (energy) is changed into the components of the universe (mass).  One day everything will return to energy.

Eccl 3:15
Whatever is has already been, and what will be has been before; and God will call the past to account.

This verse is great confirmation of foreshadows in Scripture. God “calls back the past” to play out in the future.

Eccl 12:14
For God will bring every deed into judgment, including every hidden thing, whether it is good or evil.
All Believers' sins will be forgiven.  God does forgive and forget but someone has to pay the penalty for before the sins are forgotten and atoned for.

As we look at the end of Solomon's reign, we have to reflect on his life.  Solomon started out well.  Then he began to slip away.  At first he resisted the idols of his wives, then tolerated them, and finally rationalized away the potential danger.  He didn't turn away from God all at once.  Little sins grew until they resulted in his downfall. That's why we as Believers in Jesus need to watch our step so that we do not slip away.

It seems as if Solomon was given everything, but it was meaningless to him.  However, at the end of  Ecclesiastes, he writes, "Now all has been heard; here is the conclusion of the matter:  Fear God and keep his commandments, for this is the whole duty of man." 
Way to go Solomon.  You finally "got it"!