LA/San Diego/Tijuana Tour - Suggestions?

Hi all,

I've got a 4-day weekend planned. Going from Austin, TX into LAX 3/18.
From there, planning to drive around LA, down through, San Clemente,
SanDiego and briefly (on foot) over to Tijuana, then back to LAX 3/21.

We're a couple, late 20's, young professionals. We enjoy adventurous,
thrill-type activities. We also want to see what the area has to offer
that we can't see anywhere else. Hollywood-type attractions might be
nice in LA. What's good to do in San Diego or along the rest of the
way? All we've planned so far is air and car, as we like to create the
rest of our trip on the fly, but don't want to waste time trying to
figure out what's good to do and the logistics of getting around to
them all once we get there due to the limited time.

If you have any suggestions, please post them here. I'd like to make
the most of this trip, as we haven't gotten to travel in almost a year
and have not experienced this area of the country yet.

Thanks,

Chris & Pamela J.