Moonlight hikes are hands down the most romantic walks in the Bay Area open spaces . Since most parks shut down at sunset the best solution is to find sanctioned walks led by guides who know where they are going. Googling has the drawback of landing on out-of-date sites. (would someone please tell Google to prioritize results by date!) A good place to start is the Mt. Tam Interpretative Association whose free moonlight hikes leave Pantoll 15 minutes after sunset on Saturday nights when the moon is full, on easy 2-hour/2-mile trails.
More spectacular moonlit nights can be enjoyed visiting the Point Bonita Lighthouse (check with the Marin Headlands Visitors Center) or around a bonfire on Muir Beach until 9 pm when Rangers close up shop. Occasionally there are similar nighttime jaunts in Tilden Park and Briones.