I think it is so lovely that your daughter has planned this trip for the two of you. I have been to Santa Barbara in December a few times and have been pretty lucky with weather. One year we took our girls to Hearst Castle before Christmas on a nighttime tour where they had actors in period costumes and we all loved that. I don’t know if they still do this. Lots of great wineries near Santa Barbara to visit. As others have said it can be warm with the sun out if it’s not windy. I walk and sit on the beach at home (San Diego) year round, but don’t swim in the ocean except for summer.
Others above have given you great suggestions about San Diego if you decide to change plans. There is a lot to do here and there will be many places decked out for Christmas in December to also walk around and see.
Whatever you choose to do I hope you get to relax and enjoy your time after everything you have been through.