Mission San Gabriel Arcángel, the fourth of the missions, was founded on 8 Sep 1771 by Friars Somera and Cambon. It is the home of the Campo Santo, the first cemetery in California. A large cross marks the final resting place of over 6000 neophytes as well as many Franciscan Priests.
Wikipedia has an interesting article on this mission, which discusses whether the local people were in fact indoctrinated and used as slave labor. Like so many early settlements in California, this is an issue and some missions and settlements were worse than others.
The mission was secularized with the end of the Spanish rule of Mexico. For a time in the 1850s it became the home of a rather notorious saloon. In the 1860s the chapel was used as a local parish. In 1908 the Claretian Missionaries took over the grounds and worked to restore the buildings. Today the grounds are open for all to see. There are some great exhibits including indoor and outdoor kitchens, a winery and vats for candles and more.
CALIFORNIA REGISTRER HISTORICAL LANDMARK #158 old mission site # 161
Border Field State Park is located on the Mexican Border and lies within the Tijuana National Estuary and the city of Imperial Beach, along the coast. Due to storm damage from Hilary in 2023 that contributed to the failure of an International Sewage treatment plant the park is currently closed to visitors. The day I drove up to the gate the road in was closed, but it seemed that it was open for hiking as long as you stayed on the trails. There were a few other cars parked and I could see a ranger up the road. 
