Weekly task - Play with a sandcastle at the beach.
For this task you need a toddler or preteen. You also have to unlock the beach using Sporty Orbs.
Activate the town navigation by tapping on the house icon - select the island icon. On the island the beach is available if you have unlocked it, go here and use the whistle to bring your sim to this location.
On the shore there will be at least one sandcastle. Click on the castle to get the option Build sandcastle. The action takes 2 hours to complete.