Hooray, I’m an SF local as well! The city certainly has a way of being adventurous and romantic all at once.
North Beach is fantastic, especially in the evening. Everything is bustling and there are plenty of great restaurants to choose from. I recommend Mona Lisa…it has fun (VERY) ornate decor, and every time I’ve been there, the service and food have been spot on. The Stinking Rose is probably a more famous restaurant, but personally, I find it just okay for what you get charged and the time you have to wait for a table.
If you like sensory overload/food and flowers and produce galore, check out the Ferry Building Farmers’ Market Saturday mornings. (Be sure to head inside the building for more good stuff as well!) There are usually also art stands in the plaza across the street from the Ferry Building where you can find photography, paintings, jewelry, handmade crafts and art in general.
As far as museums go, check out the websites for SFMOMA (Museum of Modern Art), the de Young, the Legion of Honor and the Asian Art Museum to see if there are any exhibits you two would be interested in. In general, the de Young and Legion of Honor are fabulous. The DY is in Golden Gate Park, which is beautiful to just walk around (SF Bontanical Garden, Japanese Tea Garden). Also, the California Academy of Sciences will be open in the park by then! I live really close to the park, and the building and space around the CAS is just…stunning. The LoH is in Lincoln Park, which is really pretty and offers views of the Golden Gate Bridge.
Speaking of the bridge, I absolutely recommend driving across and winding up to the lookout points in the Marin Headlands. It’s just breathtaking. I’ve watched sunrises and sunsets there and there are simply no words to describe how amazing it is to see the cityscape, the bridge and the bay. Just gorgeous.
Union Square is good for shopping, hotels and restaurants, but there isn’t much to do there apart from that. Anjou and Cafe Claude are excellent French restaurants there.
The Marina and Crissy Field are great places to walk around. You’re right on the water within view of the Golden Gate Bridge. Definitely picnic-worthy. There are also many cute/cozy cafes, restaurants and such on Union Street.
Cable car rides are certainly romantic! I love taking them to/from Fisherman’s Wharf (SF’s Disneyland) and Union Square. The views are spectacular, as you pass by/through Chinatown, Nob Hill, downtown, etc. And there’s something so charming about it all…
Hope this helps!