By Bus, California
Took a bus from Los Angeles to San Diego on the Greyhound.
On the way back from San Diego, an immigration officer did come on board to check for any illegal immigrants.
Between cities, the bus is an easy option. Train is more expensive. Air flights are reasonable if booked in advance on special fares. Hitch hiking has risks. And if you are not driving, the bus is always ready to go and stop where you want.
Check out following websites.
There used to be a shuttle between downtown Monterey and Cannery Row called the 'Wave'. You could by a sticker and ride all day from one place to another. I'm not sure if it is still running but it was handy and also was wheelchair accesible.
I guess the cheapest way to travel around California is the Greyhound bus. I took it from LA on Hollywood boulevard to San diego (about 4 hours I'd say). it was 26 dollars for two people (one way).
san diego has a great bus system, you can do pretty well with transportation in san diego if you don't have a car. there is also a trolley service downtown.