I'd start in North Beach, on Columbus Street, because it's great for gelato--and people-watching. You can get an Irish coffee at The Buena Vista Cafe, which is kind of their thing.
From there, walk along the water at Crissy Field. I'd recommend seeing the Palace of Fine Arts and the Conservatory of Flowers.
If it's a Thursday night, I'd go to Nightlife at the Academy of Sciences, which goes on from 6pm until 10pm. You have to go early, though, to get planetarium passes. It's every Thursday, and you can just grab a glass of wine and walk around. They have food, but I'd recommend getting dinner nearby at Sushi Bistro, on Balboa and 5th.
Crissy Field offers close-up views of the Golden Gate bridge.