As in most roguelikes, items found, salvaged, and gathered while exploring will have multiple uses. Almost any item found in the wild can be used as all of the following...
a) on their own, as a consumable restorative
b) as a component for combining / crafting within the dungeon
c) as a thrown projectile to affect a newphy or environmental feature
d) held onto, to be sold or crafted with once in town
e) traded on the MyCelial network
Here is a list of items that can be found and examples of their possible uses:
Tree-sap can be thrown as a sticky trap, or experimented with in town for the creation and upgrading of erogel guns and certain kinds of armour.
Spell-Leaves can be eaten to learn spells temporarily, thrown to cast those spells without spending power, made into tea in town to learn spells permanently, or made into step-traps to be left on the ground.
Mushrooms provide a resistance boost when consumed directly, or the caps can form containers for potions when sugar-glass isn't available.
Crystal Air can be made by fairies who have even the most basic alchemy training. It can further be refined into bottles, or used to provide an air-supply while exploring under water, through slime, through hazardous gases, &c.