A five mile mostly out and back trail (one small loop element) near the Carquinez Strait.
Start at the parking area close to Eckley Pier (worth a quick detour, be careful crossing the train tracks). Follow Bull Valley Trail up to Carquinez Overlook Loop Trail and down to Port Costa. Go through Port Costa (I'd download the route for this bit) and follow an unnamed (as far as I can tell) trail up to the top of Carquinez Strait Regional Shoreline Park for dramatic views over to the Benicia-Martinez Bridge and Mt. Diablo. The whole hike is pretty exposed but very attractive with great views and a nice town in the middle. Highly recommended.