If you're fed up with the in-laws already, or you just fancy going out for a stroll on a crispy Christmas morning, then why not head over to the south-side of the Serpentine Lake in Hyde Park for the annual Peter Pan Cup Swimming Race?
Members of the Serpentine Swimming Club have been plunging themselves into the freezing lake every Christmas Day since 1864. The 100-yard race takes place in waters reaching minus four degrees... so we don't recommend that you join in.
Note: Only swimming club members who have completed the entire Winter Series can actually take part, but spectators are very welcome. You probably have to be a little bit nuts to join in anyway -- that lake must be about minus 100 degrees in December. It's usually quite a jolly affair to watch from the sidelines, and the swimmers have been known to don a red bobble hat (to match their red nose -- it's very cold!).
There's not much else to do on Christmas morning. Practically all the shops and attractions will be shut. So maybe you can just have a nice stroll around the park afterwards. You will probably have large parts of it to yourself. Have a read of Craig's review of Hyde Park and Kensington Gardens to see what its like.
We’ve got plenty more Christmas events in London
Here are some more free events in London if you want a cheap day out
Disclaimer: Event details can change at short notice and you should reconfirm everything before making plans