I wouldn't do any kind of interval training without a tennis racket. the spanish players do tons of conditioning drills on court.
do some aerobic base training along with strength training in the pre season and then specific conditioning on court.
I think one of the best sports to go along with tennis is Ice hockey. many tennis players have been good ice hockey players, especially the swedish guys. developes foot speed, hand eye, ability to turn and change directions as well as sliding ability for clay courts.