I visited Pier 39 and Fisherman’s Wharf multiple times. During day time and at night time. I would recommend doing both. There is a great energy at night time, and many places to snack at and have dinner at. I loved the tourist shops around pier 39 for browsing. I bought cable car souvenirs there, and just enjoyed browsing through the souvenir shops. The souvenirs gets redundant, but still fun to do for a short time period. At Pier 39, you can see Sea Lions. These Sea Lions apparently showed up after the 1989 Earthquake and just near left. They make for a great attraction. Alcatraz is also visible from the pier
I would recommend walking all the way near the Apple Bees and getting fried shrimp, crab cake, crab from the vendors across from Apple Bees. The friend shrimp I had there was one of the best I have tasted. I also recommend getting a chowder in a Boudin bread bowl. Boudin is a famous Bakery in San Fran. This bakery is also near Fisherman’ San Fran is also famous for a chocolate store, Ghirardeli. You can find Ghirardeli at most stores along the pier. My favorite flavor is Creamy Almond Crunch.
Some photos below from near and around the pier: