If you want to go to TJ, the best bet is park on the US side and walk over. The line to get back to the States (in a car) from TJ is USUALLY 3 hours, sometimes longer. The wait discourages me from any trip shorter than 4 days. TJ is a better choice for cheap trinkets and Mexican blankets than seafood. TJ is not known for seafood, and the tacos are just as good in 100's of places north of the border. Look for a small, off the beaten path place that's crowded, especially a place in the lower income area of town.

Joshua Tree is terrific. If you're coming from the low desert (Palm Springs area, I-10), there are lots of hotels in Indio, Desert Hot Springs and Palm Springs, before you go "up the hill" to Joshua Tree. You should be able to make the drive from LAX to Joshua Tree in 3 hours or so, especially if you can get in a car pool lane through the city. A traffic app is worthwhile, just in case there's some sort of accident on the route that you should avoid.

It's not close or on the way from LAX to SAN, though. Anza Borrego Desert, Julian or Palomar Mountain would be a little bit closer to the route, and still scenic.