We can't really speak of what was happening in eternity past as something that happened in "time," with "time" requiring movement and change, which in turn requires creation. Therefore, time must have been created along with all else, so we cannot speak of what God was doing "before" time began--there is no proper human vocabulary for such ideas.
Also, an anachronistic nod to the 6-day creation/theistic evolution dispute shows up: "As for these 'days' [the seven of creation], it is difficult, perhaps impossible to think--let alone to explain in words--what they mean."
For example, we don't know what kind of "light" there was in the three days before the sun was made--how can we imagine a heaven lighted by anything other than the sun?
We can, however, say that this structure of creation gives us a picture of man's growing knowledge of the Creator. Just as it began dimly, it grows over time until it (as we'll see in the next chapter) culminates with rest in God.
We must not think of the Sabbath as a day of "rest" in the sense that God was tired. Rather, we must see it as completeness, as a fulfillment of what was promised and begun in the first six days. In the same way, those who are justified in Christ by faith live the lives ordained by God and finally come into the blessed rest of His eternal presence:
And so, when the inspired writer states that God rested, his words are most appropriately interpreted to mean the rest of those who rest in God and of whose rest God is the cause. And the prophecy also promises to those whom it speaks and for whom it was written that, if by faith they have drawn as close to God as is possible in this life, then, after doing the good works which God operates in and through them, they shall enjoy his eternal rest.