An elite athlete looking to infuse new activities into his training may want to choose sports that work the same muscle groups used in his primary avocation. But those of us who are just looking for a little variety in our pursuit of overall fitness are better off choosing activities that are not similar. If all you do is run and bike, your lower body gets all the attention while your upper body languishes. Adding an upper-body sport makes for more balanced fitness. Emphasize skill and movement.
Some sports are rote.
Match stressful with less-stressful. If you add one joint-pounding sport to another, you may end up having to give up both because of an injury. Better to choose new sports that are easier on your joints than the activities you already pursue. If you run or play tennis or basketball, add more forgiving sports like biking, rowing or walking. Unfortunately, aerobic exercise doesn't appear to be as benevolent toward strength programs as strength programs are to aerobic exercise.