I've only been to SF, and I don't want to bog you down with too many suggestions, so let me tell you the one thing you have to do: visit Golden Gate Park. trust me.
it's really cool. more like a huge combination garden/forest than a park. I was walking through it my first time in SF and stumbled upon a totally random koi pond with a bamboo grove and willow trees, and there's a ton of little things like that. the botanical gardens are a must-see for anyone who has even the slightest appreciation for nature as well, so many random exotic plants from around the world. I hear the Japanese tea gardens are cool too but you have to pay to get in so I skipped that; there's plenty of nature to go around in Golden Gate Park for free.
there is a ton of other cool shit to do in the city - Golden Gate bridge, Fisherman's Wharf, Chinatown, the castro district, haight-ashbury, and then other places outside the city like sausalito (very pretty) and the redwood forests, but if you only do one thing: Golden Gate Park.
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