It doesn’t really matter what you do, or where. You could attend a memorial, or just spend some quiet time at the pondering the events of 10 years ago. Check Google to see what special events might be held in the area (the Surf-a-Thon at Dog Beach was having a memorial but they moved the surfing event to the 25th due to the power outage yesterday).
I remember so clearly what I was doing and where I was when I learned about the Twin Towers, and then watched them collapse on TV. It was horrifying.
One memorial I know about is at South Cardiff State Beach on Sunday at 10 am with the Carlsbad Firefighters Association. Here is a LINK TO A FLYER about the event.