Party Time! Trophy in Yoku's Island Express
Deliver the cake to the inventors house
How to unlock Party Time!
After filling 10 postboxes, when you return to the Post-office you'll find a number of packages that you'll be tasked to deliver, one of which is for an inventor, Sin. Sin can be found in a house on the floating islands towards the left-hand side of the map, but once you arrive here you'll find that he won't let you in. However, you may spot a ledge on the other side of the house - this is the key to gaining entry to deliver the package. If you drop down to the small islands below the house, you'll find an angled platform that will launch your ball into a passage leading to the ledge, as demonstrated in THIS VIDEO from YouTube user TheMightyImp2.