My witch cabinet apothecary bottles are sold without any contents inside them. This is by design. Some folks like to fill them with the appropriate herbs & ingredients, where applicable, and I prefer to leave that to the practitioner and their preferences. Others consider the bottles decorative items for Halloween, etc. and given personal preferences, allergies, sensitivity issues, potential ingestion issues (especially in homes with children), and the perishable nature of many items, I do not wish to create any concerns. However, if you are wondering what sort of items you could put in such bottles for decorating purposes, here's a list of items you can use to fill your fantasy apothecary and potion bottles.
NOTE: If you opt for any edible options (candy, colorful soda pop, etc.) which you intend guests to actually partake of, please do not place them near bottles with contents that are not to be ingested! Remember, even common household items such as cotton balls and are not meant to be put in the mouth or swallowed. And foods such as pumpkin pie filling, relish, and tomato sauce, etc. will go bad after sitting out.
For potions & elixirs, common liquids can be used. Coffee, tea, cooking oil, and water with soy sauce or food coloring works well. Common household liquids, such as window and surface cleaners, Slime, hair gel, shampoo & conditioner, Crisco, and paint can be used. Glow water is a favorite of many.
Herbs, flour, cornmeal, dried oats, dried lentils, peas, beans, rice - alone or mixed with liquids - are easy finds. As are ashes and dryer lint.
Candy, jerky, and sprinkles for cake decorating (which now come in a wide array of colors and shapes!) are amazing & edible.
Items which may not just be laying about the house, but which are readily available include: Wax beads for candle making, moss, dried flowers & leaves, salt rocks, crushed glass, rocks (colorful or natural), stone granules (aka sand), raffia - pretty much any bowl and vase filler. Strands of rope, twine, & floss can be used for mermaid’s hair, etc. Fake worms, pig ear chew toys, plastic fingers, spiders (easy to cut them off those cheap rings!), grow-an-animal, novelty skeletons, shrunken heads, all can be displayed as specimens in bottles and jars with water or other liquid.
Some people like to include items from the compost pile or other "rotting" perishables - please note that while a decomposing cantaloupe rind makes a terrific troll ear, and leftover spaghetti makes for excellent creepy jar contents, such things are definitely "going bad" and must be kept away from children and others who may mistakenly try decorative apothecary bottle contents.