Wildfires? We may not be done yet. There are several things to think about. End of December, early January is, as elsewhere, school vacation time - usually about two weeks. typically the weekend before Christmas to the weekend after New Years. It's a high visitation and vacation time frame. Absent a winter storm at the time, the weather is much better than most of the rest of the country. Snow is generally only an issue in the high country. winter closures will be in place for the parks and highways in the Sierras and north to/through the Cascades. The west edge/coastal mountains typically don't have snow problems (unless near Oregon border?? don't know.) The big name National Parks, Mt. Lassen, Yosemite, Sequoia and Kings Canyon have some access in the winter. Significant parts are typically closed once storms begin. Which may be now, depending on how the current storm activity works out. The remaining open areas can be quite beautiful and crowded. Local lodging is likely already booked up to accommodate tourism and winter sports activities.
Likewise, you could expect crowded conditions in other snow areas, like the rest of the higher elevation parts of the Sierra and southern California mountain resort areas. If planning to make day trips to or through high elevation or snow prone areas, even day trips, etc., keep an eye on weather conditions and be aware of chain requirements. Yes, you noted you weren't really interested in snow but storms can catch folks unprepared. Tejon and Cajon Passes into/out of the L.A. area on I-5 and I-15 are at about 4000 feet and can have snow problems, chain requirements and closures during storm periods. Banning Pass between the L.A. Basin and Palm Springs typically doesn't close. Some of the other mountain passes between L.A. and Las Vegas can have snow problems during storms.
Joshua Tree, Death Valley and Anza Borrego State Park (more to the south, east of the San Diego area) are popular and it's generally good weather for them. Besides Joshua Tree, near Palm Spring is the Palm Springs Aerial Tramway. A small fire near the base has it closed now but it's expected to re-ope soon. It takes you from about 3,000 feet in a canyon near Palm Springs to a recreation area at 8,500 feet on Mount San Jacinto. Usually snowy at the top when your planning to be here but an easy trip, can be great views, and generally not a driving problem.
Tourism in the L.A. and especially Pasadena area is dominated by the Tournament of Roses. Parade and bowl game. There are some other bowl games as well, in the San Diego and San Francisco Bay areas. San Diego, L.A., Santa Barbara, Monterey and San Francisco areas are popular, and there are national forests and state parks and other rec areas which offer camping and hiking, etc.