Here's one you don't see every day - a free bus ride and on Christmas Day.
A Routemaster will be plying the 430 route between Roehampton and Putney Bridge all day, and all rides are free. Of course in the absence of other buses or tubes on Christmas Day you would have to find a way to the 430 route to take advantage....
The info from the company including a map is at https://1950s-vintage-bus-hire.co.uk/Christmas-430/
Note - although the full route normally starts at the V&A Museum and South Kensington Station, it is only running the part of the route south of the river, starting and ending at Putney Bridge. Nevertheless it could be a fun ride if you are in West London and can get there...