Harvesting Materials

Some creatures are valued for their parts, which can be used in potions, poisons, other consumables or even magic items. Generally the effect is based on the capabilities of the creature itself, as well as the specific part.
When it comes to magic items, usually they require 1 major component and anywhere from 0 to 4 minor components depending on the strength of the enchantment. This is in addition to any magical inks or other materials commonly used in enchantments, and the gold spent on acquiring the services of a proficient caster if you are not one yourself.
The amount of time and gold spent is usually dependent on the rarity of the components and the desired item, and it requires proficiency (or expertise for Very Rare items) in the relevant tool (e.g. Woodcarver's Tools to craft a magical bow).
Of course one needs to know what they are looking for, and then also be able to harvest it without damaging the parts in the process. In the case of liquid components a vial or similar is required to collect the material. The respective check required to recognize the valuable materials depends on the type of creature as listed below.
Beast/Dragon/Monstrosity/Plant: Nature check
Giant/Humanoid: Survival check
Celestial/Fey/Fiend/Undead: Religion check
Aberration/Construct/Elemental/Ooze: Arcana check

The difficulty of course depends on the rarity of the creature and the toughness of the material. Generally the difficulty varies as listed below.
CR-------------DC
1-5_________13
6-10________16
11-15_______20
16+________25

Additionally, creatures can provide rations, although some types (such as aberrations) may not be healthy to consume.
Size--------------Days of Rations (for 1 person)
Tiny__________________1
Small________________1d4
Medium______________1d6
Large________________2d6
Huge________________4d6
Gargantuan___________10d6

To harvest rations requires a Nature check with a DC based on CR using the same table above. This can only be attempted on creatures of sufficient flesh, so Oozes, Constructs, Undead and the like cannot be harvested.