Mushroom Chest

Most of what is inside duplicates a gold chest, but the mushroom-specific slot always fills, and it comes down to a coin flip between the minecart and the glowing outfit. That makes every cabin in a mushroom biome worth the walk on its own terms, which is not true of most biome chests.

Natural glowing mushroom patches underground are uncommon and cabins inside them rarer still, so collecting both exclusives usually means several worlds or a lot of tunneling. The minecart is the one to chase. It is a real mount rather than a costume, and mushroom biomes are otherwise one of the thinner places to spend underground time.