I'm pretty sure all the malls will be closed on Christmas Day. However, some of the big hotels may be your best bet if you're looking for some company and things to do.
Another idea could be a tour of the city. The most deserted I've ever seen the roads in Manila was at 6am on New Years Day. I'm sure Christmas Day would be similar. If you hired a car with a driver you could cover most of the city sights in a day.
