The Temple of Elemental Evil has a rather un-complicated crafting upgrade system. It requires you to collect various types of mushrooms throughout part 1 and part 2 of this dungeon.
Given the length of the quests involved, farming the mushrooms needed for the higher level recipes can be a daunting task, but rewarding nontheless.
This upgrade helper is only for weapons as they have multiple tiers that can be crafted upon. The armor upgrades are linear and don't have any variations.