Panjang Island is a small island just off Jepara, visited for its white sand, clear shallow water, and easy escape from the mainland. It is simple rather than polished, which is part of the charm if you want a half-day by the sea.
The island also has the grave of Waliyullah Al Habib Abubakar bin Ahmad bin Syekh bin Yahya Ba'alawi, locally revered as Syekh Abubakar. Treat that area respectfully; for many visitors it is a pilgrimage site as much as a beach stop.