"That would be very kind of you!"

He mumbles something about it not being a big deal and then tosses her a diamond pick and diamond shovel. 

She bounces happily and immediately commences flattening the landscape much faster than before. "Thanks, you're a dear."

"Well, I don’t know if I’d go that far. Watching you flatten with stone tools is, it’s just embarrassing." Wave. “I’m gonna head back now. Have fun, uh, making things flat.”

"I will! Farewell! Your generosity will be rewarded with, to be honest, the same nonsense I was going to get up to anyway but slightly faster."

“Wasn’t expecting anything else.”

And she is left alone to flatten to her heart’s content. 

This tundra is going to be SO FLAT.

She sings while she works, first in English but then cycling through all the operas she didn't include in her concert because she didn't know if anyone present would speak the language. She should probably go through her whole repertoire every once in a while to keep the memories fresh. Maybe she should be writing them down? Ooh, and she should write down all the poetry she's memorized... that can be a project for another time. Right now, she is Flattening.

Nobody else visits her that day. Tundra: flat. 

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.