I would only fly to San Diego if you actually have at least 1 full day to explore parts of San Diego itself. If not, it's not really worth the trouble to drive from SD airport to Orange County when you can just fly into the Santa Ana airport. Always consider traffic on the way to and from Orange County. It could end up being a lot longer than an hour drive.

The coast highway between LA and San Diego is not scenic except for a few places. On our honeymoon, my wife wanted to go along the coast. It was so boring she fell asleep and it took forever going thru all the local traffic.

Take I5 and get off to see Laguna Beach to see it, then get back on until Palomar Airport road in Carlsbad. From there south is somewhat scenic and the traffic is not too bad.