Night scene: GGB is not well lit at night and so is hard to see at night. If it's not too foggy (which it is this time of year), sunset on GGB, viewed from North of GGB in Golden Gate National Recreation Area is beautiful. For night city view, go on water or go higher. A good skyline view day or night is from Yerba Buena Island in middle of Bay Bridge (Go East on Bay Bridge and stay in left lane and take the YB Island exit on left side). Coit Tower on one end of Lombard St also provides a nice night view (or day view) from high up. And of course many hotels such as Grand Hyatt and Marriott on 4th Ave (not the one in Wharf) have nice city views from their top floor restaurants or bars.
Safety: San Francisco is generally a safe city. However, having lived in the City and still visiting once a month, I do not recommend walking around certain parts of the city at night. Tenderloin district (bordered by Union Square and Civic Center mentioned in previous post), Mission District (SOMA) and part of Van Ness are not safe neighborhoods to walk around at night. But Fisherman's Wharf area is safe to walk at night. Chinatown has pocket pickers and be really careful with your wallet or purse at any time.
Last but not least: It's geographically not very big but one day definitely does not do it justice - so much to do and so much to see.
You might search my previous posts on SF.