Specificity is golden in all endurance sports; therefore, there’s no replacement for swimming, biking and running if your sport is triathlon. As your level of involvement increases, this becomes more important, and to truly improve, your major focus should be the three disciplines. For elite athletes, any real cross-training is very limited, as triathlon has become close to a year-round sport that requires nearly constant focus.
But there are plenty of great cross-training options for people who approach the sport more casually, or for those limited by weather, travel or time. The best cross-training sports are those that deliver an overall cardiovascular fitness benefit, muscular endurance, coordination and a few other potential factors.
Mountain biking is a great cross-training option. Not only will it provide a sense of adventure and joy during the off-season, it also facilitates improved bike-handling skills, coordination, muscular strength and endurance, and improved pedal strokes. Many triathletes have poor handling skills on the bike, and consistent mountain biking can certainly help.
For those in colder climates, another great winter option is cross-country skiing. It incorporates an element of upper and lower body endurance, it has a strong lateral strength and stability component, and stimulates muscular coordination. While not directly correlating to swim, bike or run, cross-country skiing forces full use of the glutes and provides wonderful overall conditioning.
Another relevant sport is rowing, which is easily accessible in gyms around the country. The multi-joint exercise provides great general conditioning, endurance and power potential, and requires a certain amount of coordination and muscular synchronization.
The final activity to consider is something that I would not categorize as cross-training, but still worthy of mention. Functional strength is a key component of overall performance and should be a priority in the same way that swimming, biking and running is. There is no magic answer to functional strength, but the most useful tool I have found, which is suitable for all levels and allows progressive training, is TRX suspension training (Trxtraining.com). In the off-season, then into the full season, a progressive functional strength plan that includes this suspension training will provide overall benefits in muscular balance, coordination and power production.