These are the "things to do" I recommend to visitors who are going to be somewhere between LA and San Diego.
(These are all pictures I took back when I was all super motivated about California and brought my camera with me everywhere. Now the thing is collecting dust.)
Oceanside pier. Super duper long pier with a Ruby's at the end of it. Fun place to watch surfers and look for dolphins. Nearly halfway between Disneyland and San Diego.
The seals in La Jolla (just north of San Diego)
Whale watching - anywhere along the coast. INCREDIBLE. Tons of whales and dolphins.
Swamis Beach in Encinitas (20 mins north of San Diego). Fun place to explore when the tide is low. Lots of little critters scurrying around the rocks. Great for kids.
Heisler Park in Laguna Beach. Pretty close to Disney. Lots of beautiful flowers, landscaping, trails, benches, etc. The view is outstanding.
Carlsbad Flower Fields. Worth a stop if you're heading to Legoland or driving to San Diego (right on the way), but only stop if it's right around the first day of spring when the flowers are in full bloom. There's a lot of free parking with great views on the east side of the field (where I took this picture from). No need to pay admission to go inside.
Other places already mentioned, but I will throw in another vote...
Cabrillo National Monument in San Diego. You can see for miles. I've never been able to get a picture that does it justice.
Safari Park in Escondido. SOOO much better than San Diego Zoo. A little out of the way (30 mins from the coast), but well worth it. Animals roam free in the hills, rather than being caged up in concrete. And there's a fantastic tram tour.
Universal Studios Hollywood. The spectacular views and the 45 minute backlot tram tour are worth the price of admission alone.
And here's a lesser-known place we've visited a few times while in the Los Angeles area. Though I may get some flack for this!