Kokiri myths say that the forest is the source of all life in Hyrule. True or not, an adventurer can certainly find many useful items in the forest and inside the Great Deku Tree.
Deku Sticks
A stout staff fashioned from a bough of the Deku Tree, a Deku Stick makes a poor weapon, but it might have other uses.
Deku Nuts
The mystic fruit of the Deku Tree flashes when Link throws it to the ground, stunning nearby enemies.
Map
Within the hollows of the Deku Tree, Link can find a map of the living passages within the tree's trunk.
Compass
With the Compass, Link can better navigate the twisted interior of the Deku Tree.
Fairy Slingshot
Although the Fairy Slingshot might be used more commonly by the Kokiri to knock fruit and berries out of high tree branches, in Link's hands it will make a serviceable weapon. It is hidden deep inside the hollow trunk of the Great Deku Tree.