This is such a great series of questions, similar to what I had when starting to study Genesis. I've a good commentary on this as well ("The Genesis Account" by Jonathan Sarfati) which covers Ch 1-11 of Genesis in detail. So with some help from that here are some answers you could mull over:
How did he do the light and dark, when he hadn't created the sun yet?
Because God is light Himself, sometimes called the Shekinah glory, a light brighter than the Sun. So He is providing the light at that point.
How can he have done things in a 'day' with no sun to mark a day?
See my first answer, plus: God has separated the light from the darkness at this point, in order to have a day-night cycle all you need is a rotating earth and light coming from one direction. Since we know God created the earth on Day One, we have all we need to have a day-night cycle.
When did he do the stars and other planets?
Day Four - "And God said, “Let there be lights in the expanse of the heavens to separate the day from the night. And let them be for signs and for seasons,f and for days and years, and let them be lights in the expanse of the heavens to give light upon the earth.” And it was so. And God made the two great lights—the greater light to rule the day and the lesser light to rule the night—and the stars. And God set them in the expanse of the heavens to give light on the earth, to rule over the day and over the night, and to separate the light from the darkness. And God saw that it was good. And there was evening and there was morning, the fourth day." The expanse (or firmament) created for this was made on Day Two, which God fills on Day Four.
If he did the sea before the land, was the planet all water until 'day' three?
There's a verse in 2 Peter 3:5 that mentions something related to this: "For they deliberately overlook this fact, that the heavens existed long ago, and the earth was formed out of water and through water by the word of God". Either that or the land was submerged as is suggested in Gen. 1:9 "And God said, 'Let the waters under the heavens be gathered together into one place, and let the dry land appear.'" Hard to say whether it was all water and transformed by God into part water, part land, or the land was always there submerged.
What did he do on days 8, 9 and 10?
If you mean, what did He do after His Day Seven rest, He rested permanently from creation. So that would mean He didn't keep on creating things like nebulae, black holes, galaxy clusters, dark matter that you referred to. They were already created on Day Four. God was then busy with other things, He is working, but not creating.
Someone also asked who created God - He wasn't created. He just is. He is eternal and so does not have a beginning or an end. Beyond that I don't have enough theology or philosophy to go further!