A digestive cookie outside of games is a something that looks delicious but is actually good for you. In a game, it's a small interactive element presented as a problem even tho' there is no actual risk, the purpose of which is to cultivate useful habits in low-risk situations.
That sounds confusing but I think it's can be illustrated clearly with a few examples.
1) An ancient mural is covered in dust that obscures its subject. The PC is tempted to clean it off and reveal what's underneath because it's a mural (PCs love murals). After they move to do so, you say something like "This will require touching it with your bare hands, are you sure?" and then after a moment's panic or so, if they still wipe it off, you reveal that there was no trick or poison or anything.
2) There is a loose cat doing something adorable nearby (cats are great when you haven't prepped anything). The PCs stop and interact with the cat for a moment, and you're like "It seems hungry and dirty". The PCs debate a few options before realising the cat is not their responsibility, at which point it wanders off.
3) The PCs are in a dungeon and there is a two foot wide crack in the earth giving off vapours. You ask them how they plan to cross it, and each person takes a turn describing how they get across. When one of them is going across, the notice a gleam down in the steam. If one of them is brave enough to reach down through the vapours into the crack, they can pull up a single gold piece.
These are all very minor, somewhat silly examples, but they inculcate a practice of interaction with the environment and serve as minor opportunities to demonstrate bravery, a command of salience, and provide a moment of characterisation. Digestive cookies almost always appear in "empty" dungeon rooms in a Gygaxian sense, tho' they're also quite common in city adventures. They usually serve as a good opportunity to convey atmosphere at the same time as they make the environment interactive beyond a strict matrix of challenge or risk.
Barbie clothes are mechanically-meaningless cosmetic rewards you can give players, sometimes in the form of loot, sometimes in the form of scars or other changes. A cloak that billows in a cool way, or an eyepatch with a design etched in silver, or a beautiful but near worthless vase or a title of nobility that conveys no real power or authority or wealth are all types of Barbie clothes.
Once, in a campaign, a PC got sprayed with an alien acid, and even after the wound was healed, the flesh on his chest was translucent. Another got his skin burnt off and wore a silver skin-tight nanosuit as her new flesh. That's Barbie clothes. I think they achieve their greatest effects when they are used to soften a PC failure, or when they incentivise PC action (perhaps the cloak is on a statue in the dungeon that they have to loot it from).
This all came up in the context of talking about 5e dungeon design, as mentioned above. I'm currently playing in two 5e games (and am shortly to join a Swords and Wizardry game as a PC to keep my old-school cred intact). Because of the centrality of combat to the pacing of 5e dungeon exploration, I think 5e dungeons need a lot more "empty" rooms where there are various kinds of environmental interactivity that don't deplete resources or force agonizing decisions. Barbie clothes and digestive cookies are two ways (of several) that I introduce that interactivity without simultaneously slowing everything down with resource attrition.