StenaLine have a great day trip deal to Glasgow from Belfast for only £10 day return (Wed and Thurs) and £15 on Saturdays leading up to Christmas. Great for a bit of Christmas shopping, visiting the Christmas Markets, or going to visit Father Christmas at Hamleys Toy Shop!
I think Santa even makes an appearance on the boat on Saturdays in December....
I've done this day trip as part of a similar summer deal - you depart around 7:30am, return to Belfast for 9:45pm, and get around 4-5 hours in the afternoon in Glasgow after a coach transfer (which is in a decent Ulsterbus Goldline). From memory I've got some nice Yankee Candle and Benefit stuff from the onboard shop too.
They also run day trips to Soar at Braehead on Sun 11th for the same price, where you can go indoor skiing, rock climbing etc:https://www.stenaline.co.uk/ferries-to-britain/day-trips/belfast-cairnryan-braehead-coach
Hope it's useful to someone.