For nature & scenery, Golden Gate Bridge delivers.
When the fog lifts it's every bit the icon; when it doesn't, you're staring into grey soup with the foghorns going. Free, and genuinely awe-inducing on a clear day.
Walking and biking across the bridge sidewalks is free; tolls are charged only to vehicles and only southbound into San Francisco, collected electronically with no toll booths · goldengate.org