Carry a backpack: you’ll need to carry supplies with you pretty much everywhere unless you decide to settle on a level
Almond water: You need as much as you can. Almond water is so useful that it’s another form of currency in the backrooms. Level 4 is a great level to collect almond water from.
Food: You’ll need nourishment to prevent turning into a wretch.
Gas mask: Fog or air in some levels have negative effects.
Gloves: Some levels have substances coating surfaces that aren’t safe for us to touch.
Spare change of clothing: You’ll get dirty often, also like it’s the backrooms… everything’s unpredictable.
Some sort of weapon: Firesalt, squirt guns, and even almond water are some good weapons to have.
Blanche’s Bento Box: This item is only obtainable from Blanche. Any food stored in this bento box will always be fresh and maintain the perfect temperature no matter where you go.
One of Blanche’s books: You can find one in most backroom levels! Simply dragging your finger across Blanche’s signature will take you to the Cygnus Archive (arguably the safest level)
Blanchies: Blanchies are gifts Blanche gives to small children. Apparently they ward nightmares away.
Blanche’s Blanket: Blanche’s handmade blankets always keep you at a comfortable temperature even in hotter climates. They’re very durable (but they’re by no means a replacement for proper armor) and they don’t stain.
Worn Sack: A worn sack is highly valuable in the backrooms. You must first have a backpack that is just worn enough to make one. Apparently the process to make one is really complex, the wiki suggests you take your worn backpack to a group that makes them. The space inside of a worn sack is 700 meters cubed, and the pack will always weigh .5 pounds no matter what’s in it. There’s a ton of people who make them, here are the most reliable:
Octomari’s dog: found on level 7
The Artist: found on level 57
Zarien the Awesome Kid: found at The Hub
Coco (female German Shepard): found on level 241
Siphylis: found on level 98