It's said that half of an adventure's successes are determined beforehand in town. Actually, nobody's ever said that, but I figured it would lend some credence to my words if I make it sound like an established idea. Really though, asides from preparing spells (which only some of us get to do,) before every adventure it's wise to properly equip oneself. How can you be exactly sure what you need, though? Rope and torches are safe bets, but will you need a mirror? What about a shovel, or a crowbar, or chalk? How much easier would things be if you had antitoxin or holy water? Will there be a water hazard? An airless room? A chasm too wide to cross...?
Forewarned is forearmed, but most of the time you'll ultimately be guessing at what obstacles you'll encounter, or simply settling for the most standard useful items and hoping ingenuity will take care of the rest. After all, it's not like you can afford to haul around every single mundane tool and semi-magic item in existence 'just in case.' Wouldn't it make things so much easier if you could, say, choose which items you brought after the fact? I'm not advocating for lying about what's written on your equipment list, but rather relying on a single item that can replace a whole suite of mundane tools on a whim!
In Dungeons & Dragons, there are three cheap, accessible and extremely useful items that do nothing but reward creativity. The three of these can produce an item of your choice on the fly, whenever needed. You might not have predicted that the dungeon would have an entire passage submerged in water, but you needn't be punished because you didn't bring an auran mask... rather, you can make one.
Here, then, are the items that let anyone experience a bit of the fun of minor creation:
Nolzur's Marvelous Pigments (DMG)
Price: 4000 GP
Perhaps the most literal go-to for "if you can imagine it, you can make it." These magic paints allow you to go full Adeleine and paint real, physical objects. With the pigments, you can create any nonmagical item: not just mundane tools and equipment, but alchemical items as well.
Painting anything apparently takes 10 minutes, regardless of whether it's a pebble or a scale replica of the city of Sigil. You can't create objects of value - coins, gems, jewelry and the like end up being replicas made of cheap materials. If you want to get rich quick, you're going to need to look elsewhere. Asides from that, there are only a couple limitations on what can be painted into existence: you can only cover up to 100 square feet (or make a 1000 cubic foot item) and you can create "normal weapons, armor, and any other mundane item (including foodstuffs) whose value does not exceed 2000 gp." The problem is, these statutes are somewhat vague. Is the 2000 gp limit per item or per entire pot of pigments? For instance, you could easily fill a 10' cubic room with suits of full plate (or as a more egregious example, vials of poison,) each costing less than 2000 gp - but is the value limit applied to everything in the room as a whole? It isn't clear. If you go with overall cost, then it makes sense in balance terms, if not logic (does it really take a whole pot of the stuff to paint two flasks of pure element?) though you might have trouble determining the price for lava, bog peat, force or whatever else you decide to paint. You might need to work with your DM to determine what's the best method for what you can or cannot paint from a single pot of the stuff.
Now, with these limitations in mind, what can you make? Certainly any mundane tool, weapon or piece of armor, as well as any of the lovely alchemical items. Don't forget nonmagical goodies such as ectoplasm equipment/stabilizer (non-magical ghost touch weapons from Ghostwalk) or for even more versatility, chaos flasks (see below.) The possibilities are literally up to your imagination, as far as painting in any items you need. However, the pigments have another excellent feature that most items of creation don't - it can manipulate your environment! Is the dungeon layout a real pain? Paint in some doorways, walls, stairs, tunnels, bridges, windows, pulleys or pits wherever you like. Lace a wall, pillar or support structure with cracks and holes so it's easy to bust down. Make a hole that you can peer through or that a gaseous form can flow through. Create tactical cover before a fight. Put in a lava pit and incinerate some fool. You have to go through them all the time; why not be the one who makes an awful dungeon for a change?
Price: 100 GP
Weight: 12 lb.
Craft (Alchemy) DC: 25
Remember kinetic sand? Imagine the same thing, but you can control and shape it psychically. Sounds like magic, right? Well... it isn't. Shapesand is a naturally-occurring sort of sand that just happens to be psychoreactive: through an exertion of will (Wisdom check DC 16) you can sculpt it into any object which "serves as a normal item of the same sort." It keeps that shape permanently so long as it remains within 100 feet of you, but you can also reshape it at will with a new Wisdom check.
Sounds pretty open-ended, doesn't it? Having a dehydrated Ditto is the ideal choice for always having the right tool for the job on hand: make it into a hammer and pitons, a shovel, a ten-foot pole, a tent, a pulley - it's especially satisfying when used to make tools that wear out with use, such as hacksaws and drills. Anytime you're about to make any skill check, shape the sand into an appropriate masterwork tool for that +2 bonus. Heck, you can even make a suit of clothes: there's a beautiful irony to fashioning a desert outfit from sand. Even if the items are visibly made of sand, it's distinctly stated that they function exactly like the real thing. That said, does it mean you can turn the shapesand into twelve pounds of acid? Alchemist's fire? Explosives or poisons? Your DM is probably going to have put his foot down on these, especially since after they're used you could just revert them to sand and shape them again.
The item description also distinctly states that you can use multiple containers together to shape larger objects (and gives an example in making a small fortress out of the stuff, piece by piece.) A suit of fullplate, for instance, could be made out of 500 gp worth of shapesand: a heck of a discount, especially considering you can still turn it into any number of things and back again as needed. Any sort of exotic weapons, armor or gear could be made on demand, which could be particularly useful considering the specificity of a lot of exotic items. Since shapesand takes on the properties of whatever material it's meant to mimic, you can also take advantage of all sorts of obscure materials with useful properties. One thing to consider, though: since anyone can sculpt shapesand with a thought, if your opponent ever recognizes that your arms and/or armor are made of the stuff, they could try to reshape it with a Wisdom check of their own. Talk about embarrassing.
Chaos Flask (Planar Handbook)
Price: 100 GP
Weight: 1 lb.
As a flask containing the raw stuff of creation, just the sound of that should get you excited. By making a DC 13 Wisdom check, as a free action you can shape it into almost any nonmagical object you can think of, so long as it isn't particularly dense or heavier than half a pound. Similar to a one-item marvelous pigments, but without that bothersome price limit. You can make a Diminutive metal or stone object, a Tiny wooden object, or a Small cloth or leather object - and if you want, you can even make a (Diminutive or smaller) living creature, but that increases the Wisdom check to DC 18. Maybe you need a mouse or something as part of a Rube Goldberg machine, I dunno. Anything you make persists for a number of rounds equal to your Wisdom score before dissipating into nothingness.
Most alchemical items are nonmagical, disposable and quite small, so this is a great way to make whatever you might need at a moment's notice. Poison, of course, is also small and nonmagical, so for exactly 100 gp you can make half a pound of any poison you like. (Given that a pint of oil weighs a pound and a pint is 16 ounces, 0.5 lbs of poison is likely 8 ounces. Don't forget, however, that they will all vanish before long. So long as you have 11 or more Wisdom, that should be enough time for the poison's secondary effects to take effect.) Frequenters of the site might have already figured black lotus extract is the obvious choice, but since there's no vegetable-only limitation in place here, you can also capitalize on megapede venom (Dungeonscape) or even devastation vermin venom (Epic Level Handbook.) Seeing as the megapade venom is normally 24000 gp, that's a considerable discount.
Another crafty use for the flask would be using it to make expensive material components. You might not have the time to sell a 5000 gp diamond, but you'll be able to use it in casting raise dead! You could also use the half-pound quantity of poison to power a casting of major creation, bumping that quantity up to a cubic foot of the stuff per caster level. You will need to be working with plant materials again, of course, and summoning a djinn to make the resulting material permanent will be well worth it. The Epic Level Handbook has another devious offering in the flux slime: this transparent goo radiates a 10' antimagic field, permanently disjuncts any magic items that come into contact with it (no save) and deals 2d6 Con damage to creatures with spell-like or supernatural abilities (again, no save.) Pretty deadly to pull out in a high-stakes encounter. If you really are in the mood for a game-changer, destroying the slime (via extreme cold, heat or sunlight) will make everyone within 50 feet potentially subject to a random mutation - basically all of them are extremely bad, so don't do it to yourself hoping to gain superpowers.
The time limit on your cheated goods is a bit of a letdown, but it might be possible to keep the items in stasis for some time. For instance, the glove of storing or preserving jar might be able to hold the items in stasis indefinitely, though it's not entirely clear and your DM might still rule that they still evaporate even in extradimensional storage. Applying unguent of timelessness to an item could maintain it for as many as six hours, and quintessence is another option if you're a Shaper.
While it can be difficult to be prepared for any conceivable situation in an adventure, one can come pretty close by having a few versatile items on hand and a creative mind backing them up. For everything else, there's polymorph.