Consider the idea of checking out the Universtiy of California at San Diego!
This might seem like a strange, 'Must See Activity,' but one of the things many people like to do while traveling is to go where tourist usually don't. Also they like to meet people in the community. This is a great place for both requirements.
The campus is open and there is an interesting area near the bookstore, where anyone can purchase books, the typical university accessories, sweats, shorts, T-shirts, pens, folders, and much more.
Near the book store is a food court with outdoor seating, and just a bit further on, down the path is this, The Geisel Library. The most unusual building on the campus. The atmosphere here is very exciting and I enjoy coming just to have a coffee, but most of the time I use the library for research too.
Of course, I realize that most tourists have no interest in research, but a bit of time spent here can offer the opportunity of meeting locals as well as people from all parts of the world in a vibrant setting. Come for lunch and explore the Campus and expect that most students are willing to answer questions or just have a chat.