Huh. I saw the thread title and immediately thought 'the coast.' Guess mbriant and blessingx were thinking the same thing, too! I'd grab a good rental car and a map, and just go. Plenty of things will come up to see and do, not much planning is necessary.
I know that sounds lousy for a recommendation, but every one of my vacations is like this. I usually set a goal, like my last trip in October was eastern Oregon. I only had a vague idea of what I wanted to do, but then I got there and had an awesome time for seven days. Did that in Arizona in January, as well. Next up in the spring is the Natchez Trace. I'll fly into Memphis and take it from there. No reservations and no firm plans, save for spending a full day at Vicksburg.
The California coast is like that, as well. There's so much to see and do, you can set off driving and be occupied for a whole week. I'm not even sure a month would be enough.
Also, I can't really recommend any of the tour services. They sell you a package where they put you up in a so-so hotel in downtown LA, then bus you to Universal Studios, Disneyland, a few places in Hollywood, etc. etc. In LA, you're going to burn 5-6 hours in the bus alone to get to these places. Then, you'll have to drop $70 a head just to get into Disneyland, where you'll buy a $40 pizza and stand in line for 2 hours to get on a "big" ride. So you're going to inhale a lot of diesel fumes and wait a long, long time for the typical tour package experience. I can't imagine that being fun for anyone.
Instead, hit a car rental place and grab a convertible. Then, drive west to Santa Monica, get lunch, and start driving north on the coast. Good places to stay will turn up, lots of places to eat, and plenty to do. They will have a great time.
Also, I'd heavily recommend flying into San Francisco, renting a car, and heading up the coast into Mendocino and possibly southern Oregon, if there's time. Again, hugely fun and plenty to do.
