Ecclesiastes 3 – Skeptics Annoted Bible answered

1 To every thing there is a season, and a time to every purpose under the heaven:

(3:1-8) "To everything there is a season...." Some nice poetry and a great Byrds song. ("Turn, Turn, Turn" -- written by Pete Seeger)

2 A time to be born, and a time to die; a time to plant, and a time to pluck up that which is planted;

3 A time to kill, and a time to heal; a time to break down, and a time to build up;

4 A time to weep, and a time to laugh; a time to mourn, and a time to dance;

"A time to dance" Is dancing a sin?

Yes and no, see Gal. 5:19.

5 A time to cast away stones, and a time to gather stones together; a time to embrace, and a time to refrain from embracing;

6 A time to get, and a time to lose; a time to keep, and a time to cast away;

7 A time to rend, and a time to sew; a time to keep silence, and a time to speak;

8 A time to love, and a time to hate; a time of war, and a time of peace.

9 What profit hath he that worketh in that wherein he laboureth?

10 I have seen the travail, which God hath given to the sons of men to be exercised in it.

11 He hath made every thing beautiful in his time: also he hath set the world in their heart, so that no man can find out the work that God maketh from the beginning to the end.

(3:11) "He hath made every thing beautiful."
Everything is beautiful in its own way. Parasitic worms, cancer cells, bubonic plague. You just have to look at it from God's eyes.
(3:11) "No man can find out the work that God maketh."
Science is impossible. We can learn nothing at all about the natural world.
(3:12) Strive to do good in this life. You only get one chance.

God has indeed made everything beautiful. God didn't create diseases. Diseases are degenerated useful creatures: they had a useful purpose, but due to mutations became harmful. Read The origin of bubonic plague for more information on this topic.
On if science is possible: it is, and Solomon even engaged in it, Eccl. 1:13. But the author of the SAB quotes only a part of what Solomon says. Solomon doesn't say “No man can find out the work that God maketh" period. He says “No man can find out the work that God maketh from the beginning to the end.” Beginning to end is the important part. There will always be more to learn. There are scientific questions that cannot be answered, either because it costs too much or the energy requirements are too high. But mostly because we are not God, we are creatures with limits.

12 I know that there is no good in them, but for a man to rejoice, and to do good in his life.

13 And also that every man should eat and drink, and enjoy the good of all his labour, it is the gift of God.

(3:13) Eat, drink, and enjoy the good of your labor.

But keep in mind Eccl. 11:9.

14 I know that, whatsoever God doeth, it shall be for ever: nothing can be put to it, nor any thing taken from it: and God doeth it, that men should fear before him.

15 That which hath been is now; and that which is to be hath already been; and God requireth that which is past.

16 And moreover I saw under the sun the place of judgment, that wickedness was there; and the place of righteousness, that iniquity was there.

17 I said in mine heart, God shall judge the righteous and the wicked: for there is a time there for every purpose and for every work.

18 I said in mine heart concerning the estate of the sons of men, that God might manifest them, and that they might see that they themselves are beasts.

19 For that which befalleth the sons of men befalleth beasts; even one thing befalleth them: as the one dieth, so dieth the other; yea, they have all one breath; so that a man hath no preeminence above a beast: for all is vanity.

(3:19-21) Men and animals both die and their spirits don't survive death. "A man hath no pre-eminence above a beast ... All goeth unto one place ... Who knoweth the spirit of man that goeth upward, and the spirit of the beast that goeth downward to the earth?"
Is death final?

The reader interested in the context of a verse will quickly note the deathblow to the argument the author of the SAB makes in verse 21. How on earth the author of the SAB can ranks these verses under “Yes, death is final” is beyond my limited brainpower. Solomon makes the opposite argument actually. He argues that both man and beast die, no difference here. But what the fools, the atheists, don't see is the difference: a beast has no eternal soul, his spirit dies with him. But the soul of man goes upward to meet his Maker. Because everyone will have to give account, Eccl. 12:14. And it is clear it does not happen in this life, but it will happen says Solomon, when the soul of man goes upward.

20 All go unto one place; all are of the dust, and all turn to dust again.

21 Who knoweth the spirit of man that goeth upward, and the spirit of the beast that goeth downward to the earth?

22 Wherefore I perceive that there is nothing better, than that a man should rejoice in his own works; for that is his portion: for who shall bring him to see what shall be after him?

"Wherefore I perceive that there is nothing better, than that a man should rejoice in his own works."

But also read Eccl. 11:9.