I don't do much riding in Orange County (I'm up by LA), but I wish I did. Irvine, a town a little bit north of Aliso Viejo, has very good bike trails on its roads. It's relatively flat, too. I'm willing to bet that Aliso Viejo and a lot of those South OC towns have good bike paths. One advantage of suburbia.
I have ridden on the Pacific Coast Highway by Newport Beach before. If you are ok with lots of cars, it's a nice ride by the ocean. There are also nice strand areas on and off by the sea.
Laguna Canyon Road from the 405/5 freeway to the ocean seems like it might be a nice, somewhat hilly ride. Never rode it before but it looks good!
I can't recommend any mtb stuff, but perhaps the folks at Jax Bikes can: http://jaxbicycles.com/.
Let's see, March, early spring, you actually may have to keep an eye out for rattlesnakes on mountain bike trails. Usually around that time of year they are out more, and I think the babies (stronger poison) are hatching. Granted, I've lived in CA for more than 30 years and have only seen a rattlesnake once in the wild, on a hike in the mountains. If you do see one, just stay away from it. Watch your step. They aren't out looking for people.oh, I also don't want to encounter any snakes.
I also know that there have been mountain lion attacks in the hills in OC before, but really have no idea where exactly or what to do if you see one.