Deku Seeds are a strange form of currency in Cadence Of Hyrule that the game doesn’t specifically tell players how to use.
The game mentions that someone, somewhere will trade for them, but it doesn’t tell you who that person is or where you might be able to find them.
When you defeat a Deku Skrub, you will get Deku Seeds, and once you get tired of carrying them around, you can head over to the Lost Woods. This could take a while since maps in each version of Cadence Of Hyrule are randomized, meaning there is no guide for finding the wooded area.
Once inside the Lost Woods, explore the area and head south while looking for a tree stump that is marked with a large X carved into it. You can enter this stump and inside you will find a familiar face if you have played a Zelda game in the past.
The odd stump is home to Tingle, someone who is at the core of a lot of side quests in the Zelda series. If you haven’t completed the main puzzle of the Lost Woods yet, he will be asleep when you enter the stump. Once you finish the puzzle, Tingle will wake up and offer to exchange 10 of your collected Deku Seeds for doubling your magic and stamina gauge.
This will only pop up if you have 10 or more seeds. Otherwise, the side quest will remain somewhat hidden. Tingle will also only mention Deku Seeds after talking to him several times if you don’t give enough seeds, so this little bit of information that makes the game a lot easier can easily be missed.