The flattest tundra there ever was. Especially after she disassembles her house, flattens the small hill it was standing on, and reassembles it at sea level.

Okay. Now she can take a break and write down every poem, play, and song she's ever heard.

...this is, she discovers after a few hours, much more than a day's worth of material even writing as fast as she can. Hmm. She might have to put it aside and come back to it. On the other hand, does she really have anything better to do while she's waiting for her gear to arrive? She maybe does not. It is maybe actually the most important thing she can possibly be doing, because her memory of the world she started in is a resource she will run out of eventually, whereas the supply of landscape to flatten seems inexhaustible.

Right then, it's decided. She's just going to park in her cute little house on her expanse of flattened tundra, with all the books she can craft, and keep writing until Dream shows up to give her things. Sleeping and eating as necessary to ensure that her body continues to function. Or, well, her best guess at what is necessary; she still doesn't really understand how food or sleep work in this bizarre place.

As far as she can tell, if she's just in her house writing books, she... doesn't actually need to eat.

She does need to sleep at night.

And the next day Dream will knock on the door.

She puts down her latest book—there's getting to be quite a pile—and answers it.

"Hello!"

"Hi! I brought you the armor and tools you wanted. ...I forgot to ask if you wanted it named. Sorry. You promise that you'll tell me how you killed me after I make sure you're left alone in the wilderness for a week?"

"I promise," she affirms. "A week alone in the wilderness with my goodies and I'll tell you how I killed you."

Then she can have goodies! Lots of glowy purple goodies.

"Thanks ever so!" She puts on her armor and does a happy little twirl and then dashes off across the flattened tundra.

He watches her for a minute and then turns and heads back to civilization. 

And for a week, nobody bothers her. What does she do with her week?

Well the first thing she does is get to the far edge of her flattened tundra and then start digging an enormous pit. She's found the ceiling of the world; she intends to locate the floor, if there is one, and then count up the heights and see if sea level is consistently midway between.

There is, indeed, a floor. It's not perfectly even; it's somewhere between 58 and 62 blocks below sea level. She can build up by 193 blocks from sea level, so, not even close to halfway.

That's a weird enough set of numbers that, especially with the variation, she is suddenly having Doubts about things she previously took for granted. Like, is sea level the same everywhere, really truly? Is the ceiling height the same everywhere, really truly?

Is she going to mark two places where she believes there are separate bodies of water she's been referencing sea levels off of at various times and then build a horizontal bridge between them to check whether they're the same height? Is she going to then repeat this experiment for a separate set of distinct sea levels? And additionally build heavenstairs in all those places to check that the ceiling is the same distance away? Yes. Yes she is absolutely going to do all those things.

The sea level is the same everywhere! So is the ceiling! The numbers between sea level and the uneven floor are the same, as well.

Okay. Her curiosity about heights is satisfied. For now.

...there was a whole lot of stuff in those pits that she ignored at the time because it stood between her and her goal. She goes back and looks at the chestsful of random objects scattered by the rims of her Floor Holes. And conscientiously puts up CAUTION signs all around them, in case anyone wanders close enough to risk falling in.

Random objects: Cobblestone, mostly. Loooooots of cobblestone. Kind of ridiculous amounts of cobblestone. Quite a bit of dirt and coal. Some iron ore. Some... redstone dust, whatever that is? There's a cave with lava that she passed through in one of the holes; if she looks around in it she'll see something blueish-white and reflective on one of the walls as well as a dark black glass-looking thing on the floor next to the lava.

She does, in fact, investigate the cave, once she remembers its existence. The shiny objects are intriguing. Reveal your secrets, shiny objects!

Shiny object on the wall is diamonds! Shiny black floor is obsidian; some of it she can mine safely but some of it ends up being lava once mined.

Peculiar. Definitely worth a chapter in her forthcoming book on the wonders of Cubeland. She sets a slightly absurd quantity of cobblestone to cooking in a slightly absurd edifice of stacked furnaces, because it's just so much more aesthetically pleasing in its original form, and further investigates the mystery of obsidian. What is it doing when it ends up being lava, and why does it do that? There's some relationship between obsidian and lava in her former world, she's vaguely aware, but she bets their relationship here is very different.

Looks like sometimes there is lava behind the obsidian, and breaking the obsidian lets it flow out.

If she experiments some more, she’ll find out that if still lava touches still water, or if flowing water touches still lava, it becomes obsidian. If flowing lava touches still water, it becomes stone. If flowing lava and still water meet, they form cobblestone. 

The laws of this universe are so fascinatingly the way that they are.

Obsidian is pretty. She decides she's going to make a bunch of it, for architectural purposes. First she should do her best to safely extract all the obsidian she found here, and then she should put some water on top of some lava and see how far that gets her. Pity that the supply of lava is limited. There isn't some sort of secret law that if you sing a half-remembered opera about Pygmalion to a bucketful of lava it'll start laying little lava eggs like a weird chicken, is there? No? Too bad, but it seemed worth checking.

Is she going to be able to extract enough obsidian for a nice cathedral out of this one cave, or will she have to go foraging for more?

She can probably get enough obsidian for a cathedral out of this cave. She'll also get a really good grasp on how long obsidian takes to mine, even with shiny purple goodies.

It's for ART. She is DETERMINED.

Okay. Now, the design... better work it out in stone first, since with obsidian it would be such a pain to correct her mistakes. Tall, more square than she'd usually picture a cathedral, because it's hard to do anything with curves in Cubeland and obsidian can't even be formed into stairs to gentle the jaggedness of an arch slightly. (Why does she do this to herself? No matter. It's a challenge and she's rising to it.) Almost ziggurat-like in some respects, actually, and isn't that an interesting marriage of disparate aesthetics. She decides not to use glass for windows, but she does put in windows, tall thin ones that are really no more than gaps in the outer wall.

The stone prototype, when she's done with it, looks positively haunting. The real one is going to be even better.

She takes off into the wilderness and spends two straight days just hiking in a straight line Away, carrying all her obsidian, going over the plans in her head and in the notebooks where she wrote them. At the end of two days, she plops down a bed, builds a stone closet around it, and sleeps; then she wakes up and examines her surroundings for a good spot to put a terrifying black cathedral-ziggurat thing.

The surroundings: are wilderness. Two days’ trek away from civilization means she pretty much has her choice of biome. 

Eventually she decides on a hilly section of tundra, where she spends a while carefully considering sightlines and arranging the hills and ridges so that the cathedral is very difficult to see from far away in most directions and there's a convenient path leading in to where you'll round a corner and suddenly see an imposing monolith looming ahead. Then she builds the thing, and double-checks all the sightlines, and re-sculpts the terrain a little, and immediately starts heading back toward civilization.

Maps, right, maps are a thing, pity she left the mysterious "redstone dust" in a chest by the pit she dug it out of. She'll just have to navigate the old-fashioned way, using the sky and her memory. And singing to herself as she goes.

When she gets back, she should strike off in a different randomly chosen direction and find a different spot in the deep wilderness to hide all her written records.