Two points:

1. The things created in days 4-6 (filling the spaces separated/formed in days 1-3) are as follows: sun, moon, stars, sea-creatures of all kinds, sky-creatures of all kinds, land-creatures and livestock, man and woman.  These same things – in the reverse order – are listed by Moses in Deuteronomy 4 as things not to make idols out of or in the likeness/image of.  “…Therefore watch yourselves very carefully, so that you do not become corrupt and make for yourselves an idol, an image of any shape, whether formed like a man or a woman, or like any animal on earth or any bird that flies in the air, or like any creature that moves along the ground or any fish in the waters below. And when you look up to the sky and see the sun, the moon and the stars—all the heavenly array—do not be enticed into bowing down to them and worshiping things the LORD your God has apportioned to all the nations under heaven.”  These are created things.  They are not to be worshiped as though they were the Creator.  This anti-idolatry sentiment is present in Genesis 1.  The surrounding cultures did make idols/gods out of each and every one of these things.
2. Curiously, all of the ‘filling’ activity is on days 4-6 – except for the vegetation/plant-life, which is, as it were, tacked on the end of day 3.  The land is allowed to produce plants and trees that bear fruit according to their kind.  We see a parallel here: the ‘fruit-bearing’ word at the end of the three days of ‘forming’ parallels the command to the male and female to ‘be fruitful and multiply’ at the end of the three days of ‘filling’.  The ‘fruitfulness’ theme continues on through Genesis, and is picked up again and again throughout the Bible (and is part of the inspiration for a really great blog!).  Here in Genesis 1, the forming and filling acts of God climax in fruitfulness, and set the stage for all subsequent fruit-bearing.  In the New Testament, believers are also formed and filled by God as a part of enacting their fruit-bearing as well.