Muara Beach is a straightforward public beach at the northeast end of Brunei-Muara, a short drive from Bandar Seri Begawan. It has the things you want for an easy local outing: sand, shade, picnic shelters, toilets, and enough space to make the city feel farther away than it is.
This is not a tropical-island fantasy beach, and it should not outrank Brunei's mosques, museums, or Kampong Ayer. Its value is practical and pleasant: a place for a picnic, a walk, or a quiet break if you have transport and want to see the coast beyond the capital.