We did a 4 night cruise that left Christmas eve. As a mom, I loved it, as there was no cooking, cleaning, etc, for me!
If I remember correctly, you could visit Santa Christmas eve, and there were protestant and catholic services. I think also a midnight non-demoninational service? My kids are too old for Santa, and we don't do church regularly enough to go on a cruise, so I'm just trying to remember what we saw available. Christmas day was a sea day for us, and we just lounged around.
FWIW, we brought battery operated lights and a small tree. We did small gifts in stockings for the kids.