I will start out with what not to miss: The Sea Lions who lounge at the pier, the SF Carousel (I rode this as a kid), and a MUST is dinner at Neptune's (I ate there at every special birthday, the view is great).
This is a very touristy spot, with lots of people on the weekend, so I recommend visiting during the week during school hours if possible. If not beware that there are loads of kids and teens.
It's nice to just walk around the pier and to know that you are basically walking on something extended from land onto the water, the shops are great, some very unique shops (see LLama shop or Alpaca shop its so cool), great street performers and great food. There are also many great places to visit just walking distance of the pier, you will enjoy your time there.
You can also access many tours from here.