Mission San Jose and Mission Concepcion
I only went to see two of the four missions on the trail. Mission San Jose is the restored of the four; it is the park center. At the visitors' center there is a theatre with a short film on the history of the missions, the effects that the had on the Native Americans and how these effects have translated into the culture of the Latin American people. There is also a guided tour from one of the park rangers, which tells about the background of the missions.
Before the Texas revolution and the battle at the Alamo, even before Anglos settled in the region now known as Texas, the Spanish people came to the New World for Gold, Glory and God. In what would become San Antonio, there was no gold and no significant battles for glory, so the Spanish turned their attention to converting the Native Americans to Christianity. These missions were the first established communities in this region.
If you time, you should see all of the missions. If pressed for time, you should atleast see Mission San Jose as it has the tour and the film that describe the history of the missions. All exhibits and shows are free, so it's an extremely good value. It is very educational and well worth the time spent.