What was God doing before He created the earth?
The Bible was written for the Age of Man so it contains almost no information about either what happened before man was created (eternity past) or what will happen when the Age of Man ends at the end of the Millennium (eternity future).
We can infer from the Bible that God, Satan, and the angelic host all existed before the Creation (Job 38:4-7) but almost nothing is said about their activities. The one exception appears to be Satan’s rebellion, which probably took place in the interval between Genesis 1:1-2. And believers can also be certain of an eternal life that will extend beyond the end of the Millennium. (John 3:16) But again, no detail is given. We’ll just have to wait and see.