We have many restaurants, what type of food do you usually like? I'm not actually sure where you will see a Christmas tree other than in our big shopping mall. Remember to book in advance.
We have a nice Botanical Gardens area where you can take picnic food and goodies and if you make an event other couch surfers might join. I also think that some people could have spaces open in their homes for Christmas.
Beaches are a no-go area on Christmas day. It gets full of locals and turns into pandemonium.
We have a nice place called Camelot, where you dress up in medieval clothes and have a 3-course "feast" on Christmas day. The valley of a thousand hills is also close to it. I'm not sure about the details, but you can e-mail them or look it up: http://www.greensleeves.co.za/durban/home.html