Psalm 126 - When the Lord turned again the captivity of Zion
1 The psalmist must have been among the Jews in captivity who returned after the 70 years captivity were over, or he knew those who had. (Ezra 1:1-8)

You remember the account of Peter's release from prison in Acts 12. When he came to the door of Mary's house, the people reacted as if they were in a dream. It seemed too good to be true. Peter himself, when he was first released, imagined that he was in a dream. But the release of Peter from prison is quite insignificant when compared with the return of the captivity. Many times when I have considered the recovery of the church life I also have felt as if I were in a dream. It is so good. We never expected it, and it came so quickly and unexpectedly. Hallelujah, we are the returned captivity, and we are all in a wonderful dream. Yet, praise the Lord, it is so real! The Lord has turned the captivity of Zion. - Witness Lee
2 The verb "filled" in this verse is imperfect, indicating it is still happening. But vv 3-6 indicate that they are not happy now. I think it means that even at this present time, in their disappointing circumstances, when they remember how the Lord turned again their captivity, their mouth is filled with laughter and singing.
Also the nations are still saying that the LORD is going to do great things with Israel. (1Sam 12:24).

The Hebrew word, rinnah,  for singing in this verse is used 3 times (vv 2, 5, 6) in this short psalm.  This is the only psalm where it is used more than once. It means to sing in such a way that the singer is beside himself.

Irrepressible mirth could do no other than laugh, for speech was far too dull a thing for it. The mercy was so unexpected, so amazing, so singular that they could not do less than laugh; and they laughed much, so that their mouths were full of it, and that because their hearts were full too. When at last the tongue could move articulately, it could not be content simply to talk, but it must needs sing; and sing heartily too, for it was full of singing. Doubtless the former pain added to the zest of the pleasure; the captivity threw a brighter color into the emancipation. The people remembered this joy flood for years after, and here is the record of it turned into a song.- C. H. Spurgeon

It seems that the nations said "the Lord's to do great things with them" before the Israelites themselves had the words to describe it.

It is a blessed thing when saints set sinners talking about the lovingkindness of the Lord: and it is equally blessed when the saints who are hidden away in the world hear of what the Lord has done for his church, and themselves resolve to come out from their captivity and unite with the Lord's people. - C. H. Spurgeon
3 The first part of the verse says literally, The Lord has done great [in order] to do something with us, which is a repeat of what the gentiles say in the previous verse.
The verb "become glad" in this verse is perfect, indicating it is past and completed. This means this verse is kind of a lament. We had become glad that the LORD would do great things with us, but that gladness is past. The great things that we knew the Lord would do with us did not happen.
4 This verse is the only request in this beautiful psalm.
We should pray this for our local church and for the church as a whole.
The Hebrew word for "streams" is a river bed that dries up when there is no water, and then suddenly flows with water when rains come. (Job 6:15-20).
The psalmist is saying that our current state is like a dry river bed in the southern desert.

When we pray for the turning of our captivity, it is wise to recall former instances thereof: nothing strengthens faith more effectually than the memory of a previous experience. ...
Do not let us forget the past, but in the presence of our present difficulty let us resort unto the Lord, and beseech him to do that for us which we cannot possibly do for ourselves, --that which no other power can perform on our behalf. Israel did return from the captivity in Babylon, and it was even as though a flood of people hastened to Zion.  - C. H. Spurgeon (Heb 10:32; Rev 2:5; 3:3)
5 The godly are sowing in tears. They are sad now.
After praying for the LORD to turn our captivity as the dry river beds in the south, we need to sow. Do not think that sowing will be joyful. It may be sowing in tears because many will reject what I am sowing. This should bring me to tears, but it doesn't. To reap in joy we need to sow with much concern over those we are speaking to and praying for, as Paul was. (Rom 9:1-3). This comes by praying for them.

It is not every sowing which is thus insured against all danger, and guaranteed a harvest; but the promise specially belongs to sowing in tears. When a man's heart is so stirred that he weeps over the sins of others, he is elect to usefulness. Winners of souls are first weepers for souls. As there is no birth without travail, so is there no spiritual harvest without painful tillage. When our own hearts are broken with grief at man's transgression we shall break other men's hearts: tears of earnestness beget tears of repentance: "deep calleth unto deep."  - C. H. Spurgeon
6 "The general assurance is applied to each one in particular. That which is spoken in the previous verse in the plural --"they", is here repeated in the singular--"he." " - C. H. Spurgeon

To sow, we need to go forth. We need to go forth from our homes, friends and church.
We go forth bearing a burden to scatter the seed, which is the Word of God. Scattering is to be indiscriminate in whom we share the Word with. If we try to chose those who we think are seeking we will probably miss the real seekers.
If we sow with genuine concern for those we speak to and pray for, we shall reap, singing beside ourselves with joy..
The word "rejoicing" is the same word as "singing" in v2.
These are the great things the Lord will do with us.

He leaves his couch to go forth into the frosty air and tread the heavy soil; and as he goes he weeps because of past failures, or because the ground is so sterile, or the weather so unseasonable, or his corn so scarce, and his enemies so plentiful and so eager to rob him of his reward. - Spurgeon

For many years I was familiar with the last 2 verses of Psalm 126. I never realized, however, that these 2 verses are the psalmist's words for the return of the captivity. When we cry to the Lord to turn our captivity, the tears flow down. Many dear saints in ancient times experienced this. They wept as they prayed for the return of the captivity. - Witness Lee