Wrong forum, I'm afraid. You probably will get better results posting on the California forum:
This TripAdvisor Support forum is for questions and problems with the TA site in general.
I noticed that you inadvertently posted your question on the TripAdvisor Support forum, so I have moved the topic on your behalf to the California forum. I hope you find the information you need to plan your upcoming travels.
TripAdvisor Support Team
We've visited all 21 Missions over the years. In CA our 4th Graders often have to choose one for a report during the year, so families are visiting ones close to home.
Santa Barbara is one of my favorites for its beauty and location. If you are starting your drive in San Diego that would be a good place to see one. My other favorite is in Carmel. Some you will see a sign off the freeway and can make a quick stop, like Ventura, CA.
Thanks to everyone for the responses including getting this to the right forum. We're under the impression that San Luis Rey near Oceanview is still being renovated. I'm thinking of going to San Juan Capistrano and maybe one more.
Since you are starting in San Diego you might as well go to the first one which is right there!
IF you are interested in history then don't forget Cabrillo National Monument(also in San Diego) where the first explorer(Cabrillo) set foot here
in the 1542.
La Purisima Mission is very interesting as it shows how the mission was a settlement and not just a church. It's a California State Park and is just a short detour off of Hwy 101 which is the road between LA and San Francisco.
I would also stop at the Santa Barbara Mission which is known as the "Queen of the Missions". It's one of the most visited places in Santa Barbara.
And, if you visit Carmel, the mission there is also very worthwhile to visit:
My favorite is San Antionio de Padua. It is more remote but the remoteness makes me feel closer to the place.
Mission Basilica San Diego de Alcalá is at the east end of Mission Valley in San Diego and dates back 244 years, though the current church there is only 82 years old. You can visit any day, but I would recommend attending the 12 noon mass on Sundays to hear the choir which is outstanding and sang for the pope a couple years ago. You do not have to be Catholic or do anything to attend the mass, you can just sit there. Afterwards take the short self-guided tour through the church, rose garden and small museum.
Also in San Diego is the Serra Museum, which was the original location of the mission, at Presidio Park above Old Town. There is a small fee for entering that museum.
Mission San Juan Capistrano is famed for the swallows who return to it every Saint Joseph's Day, March 19. Mission Santa Barbara is a pretty one, located right in the center of Santa Barbara. The Carmel mission is also pretty. I also like Mission San Juan Bautista, though visiting that one would be a detour off your main route.
As a native Californian, I’ve somehow missed a few of the missions. I was at San Antonio de Padua just last week, and San Juan Bautista and Soledad last month, but none were first visits. The original El Camino Real (“Royal Road”) roughly follows Hwy 101, but not exactly. So San Antonio is accessible but not on today's main highways, either 1 or 101. It is near one of the rural roads that connect the two, Nacimiento-Ferguson Road, which is steep and winding and probably should be avoided in stormy weather.
From 101, going to Mission San Antonio is a detour near Bradley onto a county road, which rejoins 101 near King City. It is on a U.S. Army base and you must have driver’s license, registration, and insurance certificate. It’s a rural area, but the detour will only add about 12 miles. If you take the Coast Hwy along Big Sur to Monterey, this mission will not be on your way.
Santa Barbara is called "Queen of the Missions" because it is very beautiful, and it's well preserved. It, like San Diego, San Fernando Rey de España, San Francisco de Asis (Mission Dolores), San Rafael Arcangel, and several others, has had a major town or city grow up around it, and so the environment you see it in is totally different from what it was when Father Serra or the other Franciscan friars were here.
Many missions are in or near small towns (Purísima Concepción, San Miguel Arcangel, Soledad). Often, these little missions have a more historic, peaceful, spiritual feeling than the more celebrated touristy ones, but you’re less likely to visit them, at least if you take the coastal route for much of the way. Assuming you’ll want to take Hwy 1, I’d consider San Diego de Alcala as the first in the string; San Juan Capistrano; Santa Ines in Solvang; Purísima Concepción in Lompoc; Santa Barbara, San Carlos Borromeo (aka Mission Carmel); and Mission Dolores. In Los Angeles, there is San Fernando Rey, perhaps the ultimate example of a mission totally immersed in a modern city.
To get a more real sense of California history, I would pick San Juan Bautista (a few minutes off 101 north of Salinas but out of our way if you are taking Hwy 1 ll the way) or San Francisco de Solano (aka Sonoma). They are in historic districts, so instead of being lost among modern homes or businesses, they are surrounded by 19th century buildings, remains, or re-creations. SJB has the added attraction of being literally on the San Andreas Fault, and you can walk behind it and see the displacement between the Continental and Pacific plates.
Especially at SJB and the adjoining state historic park, you can see how closely the first inhabitants, Spanish colonists, Mexicans who succeeded Spain, and finally the American settlers interacted, often conflicted, and impacted each other. The missions are beautiful. They and six presidios were the vanguards of Spain’s multi-pronged objective in the New World—God, gold, and glory. While the Franciscans had lofty ideals to bring souls to God, and did the best they knew how with the understanding and viewpoint of the 18th and 19th centuries, the original folks often fared poorly and mission history isn’t always a pleasant picture. One of the saddest sights at any mission is the Indian graveyard, where you’ll see unmarked mass graves of thousands of people who died from various causes after the first colonists came.
Many visitors to California miss out on seeing a mission, often because they aren’t on a main travel route and they aren’t as publicized as other visitor sites. But they give you part of the picture of how California came about. For visitors who are Catholic or want to attend Catholic services, many of the missions are also active parishes that have regular Mass schedules.