This mission is off the beaten track through Oceanside, but well worth a diversion. It is old, it is large, and it is great to look through.
The mission has much to offer: a large grass area with picnic tables, a view of the adobe ruins of former military barracks, a mission church, a mission cemetery (but not dating to the mission founding), a really professional and wonderful museum, the state's oldest living pepper tree, a large gift shop and a cute little coffee shop and patio area. The most offerings of any mission I have been to so far.
The best advice I can give you is to take your time and enjoy all the exhibits and look everywhere you can - the tree came to my attention as I was heading to the restrooms, and the café was found that way as well! Never know what you'll come across . . .
Allow at least 2 hours for a decent visit, although I tend to really engage and spent about 3 hours here. But as mentioned earlier, well worth a trip off the beaten path.