I am a native from Northern California so as far as "fun" I recommend: San Francisco, wine country (Napa/Sonoma) or Lake Tahoe. However, if you're looking for more of a beach scene, then definitely San Diego.
Santa Barbara is beautiful and very scenic/romantic! You can rent bikes and cruise along the beach, walk along the pier, the restaurants are great and you don't have to travel far to do some wine tasting. I may be biased because that's where we were engaged lol.