From where the race started, walk down. Instead of crossing the bridge to the right leading to the high tower building, climb up the structure to the left. Destroy the golden wall with Fire Spinjitzu and do the wild water race.
Now go towards the swamp. But not the story path, stay on the right. You can climb down onto a little platform and jump across to the right, where the golden box is. Grapple the little house at the top first.