It really depends on your budget and what you want holiday wise. Laguna beach (south of la in Orange County) and la Jolla ( just north of San Diego proper)!are lovely, walkable towns but hotels are expensive there. If you're happy with a house/apartment you could try Airbnb as they tend to be cheaper than 2 hotel rooms. If you want warm water you really need to be between LA and San Diego. Pacific or mission beach in San Diego have reasonable priced options, based here you're close to the new sea world water park (actual water park not sea life) as well as actual sea world, Legoland, San Diego zoo and wildlife park. It's a 4 hour drive from here to Vegas or you can make a daylong scenic drive through the salton sea, Joshua tree park and the mojave dessert preserve. La Jolla is worth a visit if you're in San Diego even if you're not staying there, there's a beach called children's pool where the seals and sea lions hang out, you can get really close to them. Newport Beach is just north of Laguna, (also Orange County) about 1 hour south of LA and has cheaper options than Laguna, it has a lovely long beach with lots of activities for the kids, as does Huntington Beach. Further north towards SF, you have Santa Cruz (very reasonable price wise) and Monterey. The sea is cold here though. Based in Santa Cruz you are about 1/2 hour drive from Henry cowell redwood park, you can see the giant redwoods there and even go zip lining in them. There is a lovely walk in the woods over a railway track to a swimming hole surrounded by the redwoods, with rocks to jump off if you want an unusual swimming experience. There are a couple of waterparks close to Santa Cruz too. Wherever you stay I'd recommend a whale watching trip, we went on one from Dana point, south of Laguna with an underwater viewing area to watch the Dolphins zooming alongside the boat. The area around Laguna is known for its mega pods of dolphins and they're amazing to see. August is blue whale time too, we were lucky enough to see a mother and baby. I'll shut up gushing now before I bore you to death! Any more info, just ask.