What was God doing before creation for all eternity?
This question is asked sometimes with an idea that it would absurd to believe that God was doing nothing and therefore the whole idea of an eternal God creating is unreasonable. This has been argued y P.C. W. Davies according to Dr. William Craig:
“Davies argues that God can be neither temporal nor timeless. He says that God cannot be timeless because God, as described in the Bible, is a person; but persons are inherently temporal in nature. They act and react, they're conscious beings who deliberate and anticipate and remember. They think about things. They intend to do things and then carry out those projects. All of these are temporal activities, and therefore if God is personal, as the Bible claims, God cannot be atemporal, or timeless. On the other hand, says Davies, neither can God be temporal. For if God exists in time, then He is subject to the laws of relativity theory, which govern, space and time, and therefore He cannot be omnipotent because he's under the laws of nature. So the theist is confronted with a dilemma’”
How can we answer this question?
First, most humbly perhaps, we would have to say that we don’t know since God has not told us clearly what God was doing.
My ignorance does not mean that the answer is either “nothing” or absurd. It just means that I don’t know. Just because we could not imagine an answer to this question does not means there is no answer. So agnosticism concerning this question may be the most humble response.
Second we do know that in the Christian perspective of “One God who exists eternally in three persons” that the three persons of the Trinity were in loving and respectful relationship, conversation, and dialogue would fit the outlook of the Messiah Jesus.
Gospel of the Apostle John 17:5 NASB - "5 "Now, Father, glorify Me together with Yourself, with the glory which I had with You before the world was."
The idea of glory is the communication of truth and beauty. So this is an indication that the Father, Son, and Holy Spirit were active in relating and communicating before creation. This would also mean the planning of creation, redemption, and judgment as well, would have been part of the conversation. This seems to be in the thought of the Apostle Paul
Letter to the Ephesians 1:3-4 NASB - "3 Blessed be the God and Father of our Lord Jesus Christ, who has blessed us with every spiritual blessing in the heavenly places in Christ, 4 just as He chose us in Him before the foundation of the world, that we would be holy and blameless before Him. In love"
So this would seem to be part of what was occurring for God before the creation of the world.
This problem is made more difficult because of God’s relationship with time. Some would say that God is “timeless” or outside of time entirely or is the Maker of heaven and earth relationship to time on of “divine temporality”, in which the LORD of creation has always existed in eternal time? 
This topic is worth its own discussion and Christian thinkers differ on this issue. The scripture would seem to support a type of “both/and” approach in which God before creation was “timeless” and after creation entered into the realm of “time” as part of divine involvement in creation. The prophet Isaiah reflects on this when he says:
Isaiah 57:15 ESV - "15 For thus says the One who is high and lifted up, who inhabits eternity, whose name is Holy: "I dwell in the high and holy place, and also with him who is of a contrite and lowly spirit, to revive the spirit of the lowly, and to revive the heart of the contrite."
Isaiah 57:15 NASB - "15 For thus says the high and exalted One Who lives forever, whose name is Holy, "I dwell on a high and holy place, And also with the contrite and lowly of spirit In order to revive the spirit of the lowly And to revive the heart of the contrite."
It should be noted that bible translations are divided on this philosophical issue of God’s relationship with time and this is reflected in the two translations used above.
What is clear that Isaiah is saying that God exists in a high and separated place not being part of creation and yet has decided to be deeply involved with those who are seeking truth and struggling with their moral failures. He is both “far away” from us in being the only being that has existed forever and is not created, yet near us because of God’s concern and care for us.
Now if we take the timeless view of God with time being part of creation, then the question of what God was doing before the creation of the world is a problem of applying a category of “before” which does not actually relate to God. Being “timeless” all that happens in time happens as a singularity or as an eternal “now”. Therefore in this view there is no “before or after” only “NOW”. So then the question simply does not exist for a being that is “timeless”.
If God simply “lives forever” within time then the question can be explained in part with the discussion of the Triune God being in dynamic and ongoing relationship and dialogue with each other. The Christian view of God as Triune would be superior at this point in answering the questions over a simple view of God, which did not include a Triune conception of God’s nature.
Now if God simply “lives forever” we would also have to fall back on the humble agnosticism which simply says that we can only really know things about God if they are revealed to us and without such revelation any theories we may hold would reflect mere wild speculations.
However, not knowing is not the same as demanding that some type of impossible paradox exists which would demand we reject faith in a personal and all powerful God. To make an argument from silence, would be to not honor the lack of divine revelation we have on this topic or be valid.
Therefore, we see that we can say that the Triune God was in relationship within the Godhead and that there is not impossible paradox set up by those who would raise this question.
Here is a discussion on this topic that looks at how we could resolve these issues.