I just did the same thing. #1 suggestion would be San Jacinto Peak via any number of trails. For a longer hike start from the Idyllwild side, for an easier cruise up, take the Tram from desert station and stay in round / tamarack valleys. It has Sierran Views, a running spring pipe, pit toilets, established sites and a meadow full of deer. The view from the summit is awesome. Or, for more coastal adventure, ride the ferry over to Catalina Island and use the trans catalina trail staying in Little Harbor, Blackjack and other seaside campgrounds. Well established trails and even a shuttle if she doesnt dig it. San Gorgonio Mtn has good trails also.... and if it's winter... you must try Anza Borrego Desert on the Collins Valley, Coyote Canyon side. Year round stream, willows cottonwoods and the best sunrises, sunsets and stargazing going. All within San Diego County.
Hope these Ideas spark something! I can send a link to some photos of San Jacinto from a couple weeks ago. Took my 8 year old and 7 year old up there. Just send me your email if that 's the trip you're most leaning towards later.