Escape Game: Aloha

Story-related and cannot be missed.

You will unlock this when you unscrew the pylons in front of the sandcastle (screwdriver needed; it's in the kiddie pool that needs to be popped open by the thumbtack), then approach the sandcastle and click on it until it collapses. Underneath the sandcastle is a watermelon, which needs to be dug out with the red scoop that should already be in your inventory by this point in the game (if not, it's under the table with the drinks on it).