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

Wesley's Notes

3:12 Them - In creatures or worldly enjoyments. To do good - To employ them in acts of charity and liberality.

Jamieson-Fausset-Brown Bible Commentary

12. in them-in God's works (Ec 3:11), as far as relates to man's duty. Man cannot fully comprehend them, but he ought joyfully to receive ("rejoice in") God's gifts, and "do good" with them to himself and to others. This is never out of season (Ga 6:9, 10). Not sensual joy and self-indulgence (Php 4:4; Jas 4:16, 17).

Matthew Henry's Concise Commentary

3:11-15 Every thing is as God made it; not as it appears to us. We have the world so much in our hearts, are so taken up with thoughts and cares of worldly things, that we have neither time nor spirit to see God's hand in them. The world has not only gained possession of the heart, but has formed thoughts against the beauty of God's works. We mistake if we think we were born for ourselves; no, it is our business to do good in this life, which is short and uncertain; we have but little time to be doing good, therefore we should redeem time. Satisfaction with Divine Providence, is having faith that all things work together for good to them that love him. God doeth all, that men should fear before him. The world, as it has been, is, and will be. There has no change befallen us, nor has any temptation by it taken us, but such as is common to men.