How to unlock the beach and the seaside street ?

To access the beach you need to be at level 25 and complete the 3 quests: Digging Sideways starting at the Cave with Darthy. Once completed you will unlock the beach in the menu/map.

Once the beach unlocked you will be able to unlock the new Seaside Street. You need to complete the 3 quests called: Pretty Sunny and once completed you will be able to access the seaside street from the menu/map.