If you have limited amount of space and time to get an off-court workout, I would recommend looking into resistance training.
For tennis, it is much more helpful in building flexibility and tennis-specific movements than free weights can alone. It is also great as an injury-prevention tool off the court.
Check out this YouTube video review from Tennis Channel!
I would recommend this type of suspension training over normal gym workouts because there is less impact on your joints such as knees and ankles. It allows you to be creative and carry it all with you in a small bag, making it very portable and efficient.
For myself, I use a system of resistance bands for my indoor training, but currently on the market is a high-quality workout system called TRX Suspension Trainer.
**Here are some Deals on some TRX Training Bundles at FitnessAnywhere.com