After many years of thinking about it, I finally got round to visiting Treasure Island. Lying between San Francisco and Oakland it was originally created from fill in the 1930s. The island was mainly occupied by the Navy and has only recently been donated to San Francisco. Nowadays it's a mix of residential housing, hangars, fields, and derelict places. My favorite landmark was a "Genetic Reclamation Area" sign.
Why go there? For spectacular views of the Bay and, if you're willing to explore, you can drive down to the base of the Bay Bridge and see some of the impressive construction work going on.