There are many items in the long trade sequence, but only a few actually lead you to be able to continue on a path towards a specific dungeon. The Bananas are that very item that we covered how you can get in another guide, which you will need to get into Kanalet Castle, which eventually leads you to get to Key Cavern. You have to travel even further away from Mabe Village for this trade, so this guide will detail just where you need to go.
After finishing the second dungeon in the game, Bottle Grotto, you will have access to the all powerful Power Bracelets that let you pick up heavy objects. This will now allow you to travel west of Mabe Village by removing the rocks, which is what you need to do. Start going east from here and then go south when you get a chance along a narrow strip that winds around eventually near what will be Key Cavern, which is marked by a key slot.
Keep going south and speak with Richard in his Village, who sends you on a quest to find five golden leaves. This sends you towards Kanalet Castle, which is to the northeast. As a result, keep going in that direction and you’ll eventually see a castle entrance that is sealed off.
To get in the castle, you can head to the right and you’ll see Kiki the Monkey. Speak to Kiki the Monkey and they will ask for your Bananas that you have. After doing so, a bunch of monkeys will build a bridge across for you to continue the quest, but also leaves behind the next item in the trade sequence, a Stick.