Since San Diego and Santa Barbara are roughly 2 hour drives from LA they are do-able as day trips. I did it a handful of times when I lived in LA. Bear in mind that choosing to go as a day trip means:
- You have to get up & leave LA very early if you want to get there before noon.
- After you do whatever you do during the day, you still have to drive a few hours home.
- This works okay when there are one or two specific things you want to do. It does not work as well when your goal is to "experience the city" or "look around and see what's there". (If these are your goals you really should get a hotel in town.)
- Factor in driving delays due to commuter traffic M-F and weekend midday traffic.