If a lot in the "beach" themed area of Vacation Island has a rental shack, Sims can rent a sandcastle kit. Once the kit is rented, the Sim who rented it will go somewhere on the lot and begin building a sandcastle, but the player has no control over where the Sim will go. Sims with higher creativity skill build better sandcastles.
The player can direct other family members to join the activity, in which case the renter's creativity skill seems to control the quality of the sandcastle. A common problem on the Reclaimed Public Beach is that Sims will build sandcastles in spots that other Sims who want to join in can't get to.