I just learned that all the bits and bobs I pick up can upgrade my items.
What are all the items that I can upgrade? I want the best of the best, after all. What do I need to do each upgrade?
There's a list of what every item requires for upgrade on this page.
The short list is:
Additionally, you can upgrade the Beetle, the Bug Net, the Bow, the Mitts (although the upgrade is part of a dungeon treasure, and not via collectable items), and the Slingshot.
There are also additional wallets available for purchase, although calling them "upgrades" might be a stretch.