It is very difficult to use only the information available in the text of Genesis and try to make a sound determination or “scientific” examination of it in a way that makes sense. Realize this is a text written LONG after these events would have taken place. Realize also that the “things” created in the story are things that were major divisions in the Hebrew culture at that time. Earth, water, plants, animals, people, etc. You don’t see cars or football teams in there because they didn’t exist at the time of the writing; only the important divisions at that time are represented.
The writing style is not the same as later, historical texts. That could be because the writer(s) only knew this style and used it, or it could be that they intended it to be broad in scope, meaning each “day” represents major milestones in creation. The difficulty you get into when you try to apply a “gap” theory, where each “day” is said to be a millennium, is that you end up with plants created in one millennium, but no light until the next millennium. To use a gap theory, you have to pull back from any kind of literal application of the events themselves (meaning you have to say plants and light came into being close to the same time) and relegate them to figurative, major milestones.
Created close together, but the “epoch” assigned to each is far apart only to distinguish them as different from each other.
The significance of the whole story is that there seems to be a certain “pecking order” in the creation itself, where lower forms of life are created first, man - the highest form on Earth - created last. Then later in the text man is naming the animals, signifying a higher importance in the pecking order. Then man is given dominion over the creatures and the Earth as a way to prosper and multiply.
It is also difficult to rigidly enforce the idea that ALL of X is created on this day, ALL of Y is created on that day, when you have God with a created Adam creating some things right in front of Adam. This suggests, at least, God demonstrated His creative power in front of Adam so he could know of the things that happened in the “days” before he was created. Not a full “second” creation, but merely a demonstration of the essential pieces of the story that preceded him.
Interesting to note that the apparent “two creation stories in Genesis” problem we still see mentioned today was the same problem that helped create and fortify much of the paganism that prevailed in early Jewish history. It led to ideas such as their being a different, first “Eve,” and other persons or demi-gods created first that somehow didn’t work out, thus God had to “re-create” it a second time. It is these pagan theories that most closely match up to the polytheistic beliefs of the Babylonians and Assyrian gods, and that is why some scholars insist that Abram did not begin with a monotheistic religion. I don’t believe that because there is no good evidence aside from the idea that “everyone else did it so Abram must have done it too.”