There are four (Mission Concepcion, Mission San Jose, Mission San Juan, Mission Espada) Missions which mark great history and are pretty huge. A lot of history to read about and architecture to capture. If you don't have enough time to visit all of them, my personal favorite was Mission san Jose. I just loved the architecture. Also, I collect coins as souvenirs from all the places I travel to, perhaps I got myself one from the Mission too! a memory to hold on to. The entry to the museum is free and the bus stops are right outside the missions which makes it super convenient to travel.