If you want to make the most of a little extra time to train, while also enjoying the festivities and time with family, you must first stay fit and healthy to do so.
It's easy to indulge on Christmas food, and I wouldn't tell my athletes not to enjoy themselves, just don't neglect the vitamins and minerals you usually would get with a balanced diet. So be sure to keep on top of your fruit and veg while also enjoying the pigs in blankets which as lovely as they are, aren't exactly nutritionally dense.
A lot of club sessions stop over Christmas, but you may find you have more time to train. For the normal time-crunched athlete, high-intensity intervals are a great way to keep fit and get some performance gains in a small window of time. However, with the ability to sneak these sessions in all year round, with the additional time Christmas brings, I often recommend volume is the best thing you can be doing at this time of year. It may be portrayed as old school, but steady aerobic runs/swims/bike rides still massively benefit the aerobic system - it comes to forcing the adaptions you need to become a better athlete.
While this would be ideal, it may not be the easiest thing to get past your other half, but this can now be achieved indoors. Turbos and treadmills used to be for the ultra keen, or severely time-restricted. Now they make up a big part of many athletes training when the weather is a bit miserable. And the big switch has come mostly in the form of Zwift. This works for running or cycling (sorry it's not linked up to endless pools just yet).
For those who don't know, Zwift links up to your smart trainer, power meter, foot pod or other compatible devices and transports you to the tropical island of Watopia (or the slightly less tropical Richmond park depending on the calendar). The added focus this brings to your riding is game changing. A previously dull 30-minute slog can easily become a 90 minutes plus game of racing people online, either in the official events or just in the open riding mode, while clocking up XP points to allow you to get better bikes or cooler kit. I would definitely recommend using any extra time getting Zwift set up, and trying some of its group rides for some added motivation.
This may sound cliché, but use the time to do the sport you love in a more stress-free environment if you can. Explore new routes, do something different like mountain biking or fell running. Providing you're keeping active it's an excellent opportunity to create some training adaptions in some entirely different ways and give your brain that mental break it needs, allowing you to go fresh into 2020.
I wouldn't say I like these as a coach, positive changes in behavior take time and is not a quick fix. Suddenly a number of the year changing does not mean you can suddenly become a new person. We all have things we can work on; eating healthier, sleeping more, training more. However, attempting to double your training while halving your calories over two weeks is only benefiting your local physio as you turn up injured. And that's if you get that far without giving up.
So while you may want to think about a few improvements, you could make to aid you being a better athlete. Why not just start them now? Why would you need to wait till January for a positive change, but start anything small, and consistently. That is the key. There is no quick fix in sport. If anything the best new years resolution for a lot of people would be no more resolutions, fads, radical changes, but to focus on consistency within their lives, training, and creating positive changes gradually.