One of the popular but still fairly under the radar day trips around San Francisco is the ruins of the Sutro Baths. The ruins are indeed ruins, but they are quite modern (especially for ruins).
Opened in 1890, the baths were at the time, the world's largest indoor swimming pool. Developed by wealthy entrepreneur and at one time San Francisco mayor Adolph Sutro. Less of a bathhouse and more of a large salt water swimming pool, the Sutro Baths had trouble staying afloat financially due to high operating costs.
Maintaining a pool of that size, even a salt water one, was not cheap. The baths were closed, and shortly after their closure in 1966, the baths burned down in an act of arson.
Insurance money was collected, and everybody moved on. But the ruins of the baths remained, and still remain to this day. It is now a popular spot for urban explorers and adventurers.