A great midweek hike in Marin is the Marin Headlands. The short Tennessee Valley Trail takes you from an inland equestrian center along a one mile trail to the Pacific Ocean.
The trail sits in a valley floor runs alongside a fairly decent size creek. The combination of bog plants, valley grasses, hillside chaparral, and windswept trees offers fantastic scenery. There's plenty of opportunities to take pictures and soak in the vistas on a bench.
That's all before arriving at the beach. It's a narrow beach with soft sand and loud crashing waves. The trail is well marked but it's easy to get lost in the surroundings!