Skipping past the history lesson, I'll just say that the California missions are some of the most significant historical/archaeological/architectural sites in California. In some ways, the history of the mission is the history of California, and the four largest cities in the state grew up around four of the 21 missions. They're also responsible for there being a wine industry in California and, therefore, the United States. They're responsible for a lot of other things, too, not all of them good.
My wife and I have plans to visit all of the mission sites; so far, we've visited five of the 21, three of them back around the beginning of August. Here are some pictures from those visits. Without labels or anything. Sorry. No, I'm not. If you're really curious, you can look up the missions and see if you can match my pictures to the appropriate one. Sounds like fun, right!