To get the Beach Sign, you need the Pool Toy rewards from the Casey Skatie Event.

Once you start this Story Event, you have 3 days to level up as much as possible and collect the Pool Toy rewards, which need to be merged up to Inflatable Dolphin Level 6.

Then you feed those with Butterflies and Moths of different levels in order to be activated.

Inflatable Dolphin Level 6 can drop Flip-Flops Level 1, which you can merge up to Level 12 to create the Beach Sign.