Your performance for the past year - whether you're professional, a student, or a recreational dancer - will have areas you want to improve, techniques you want to develop, new feats you want to achieve. Maybe you're getting bored with one aspect of your training. You have a class that's become routine but doesn't seem to be developing you. Maybe you're getting frustrated at a particular step or technique. Maybe your grand jetes aren't high enough or you're still falling out of a triple pirouette. Maybe you want to be stronger, or faster, or more powerful.
Dancers tend to train in dance, and that's it. Exclusive dance training isn't necessarily a good thing. Your muscles are performing the same actions and loading the same strains every day; for any further developments to be made you need to change your training. Some companies, and dance schools, have started incorporating mandatory strength and conditioning training into their dancers' schedules, which can be nothing but good news. Others incorporate pilates or the Feldenkrais Method as programmes, the merits of which I remain unconvinced of. For most dancers though, training consists of a mixture of technical class, pas de deux, repertoire, occasional body conditioning and more technical class. There seems to be a fear of pushing the body into any movement patterns not akin to those performed in class and onstage - I remember being told by my teacher to stop swimming, as the movements of my legs in the pool would be so contra to those performed in class that I would be exposing myself to horrific knee injuries. There was nothing wrong with swimming and dancing. She was regurgitating what she had been told by her teacher, and what she in turn had probably been told by hers. Any exercise performed safely is good for your body.
If you're in full-time work or training, your physical workload is hard enough, and you aren't going to be able to drop a technique class to take up something new. So instead of looking at cross-training as extra work, think of something that you either really want to try, or something that you think will benefit the areas you're looking to improve. This could be anything - climbing, running, rowing, athletics, martial arts, tennis - they're all going to make you use your muscles in different movement patterns, will develop different areas of strength and increase your movement vocabulary. Many professional football teams, most famously the Brazil national squad, cross-train with ballet. It improves their agility and co-ordination. Reverse that and equally football could increase your speed, your spatial perception and aerobic fitness.
If you pick something you enjoy or genuinely want to do, it's recreation, not work. Do it once a week, make it social, don't necessarily think of it as extra training. Don't shy away from pushing yourself into something new, your fitness levels will improve and you'll be able to recover from a plateau in your training. Make a decision when you start the new year to expand what you do and take on something new. It really doesn't matter what it is; as long as it is something different and you want to do it you're already going to be improving your movement vocabulary and physical skill. Regardless of where you're making these developments they will always translate into improved physical prowess, which in turn makes you a more able dancer.