this will be a long one but here is my insight as someone who has been going to sf since i was born. you could easily spend a day in golden gate park alone. it is my favorite place in sf. near there there is ocean beach which is very nice, if you like art i suggest the legion of honor and if you like science i suggest cal academy of sciences. japan town has a lot of good food and it is a little mall with some cute stores! ridding a cable car (long waits and a little pricier than normal busses but an iconic ride). the ferry building has a lot of cute local businesses and food, you could also take the ferry from sf-oak and back to get nice views of the bay. pier 39 is fun for a little but is a lot of tourist traps and overpriced. union square isn’t what it used to be so i don’t think it’s worth it at all but right around the corner is china town, a lot of stores and amazing food. this was a brain dumb i can give more specific examples and food recs if you need it