I'd suggest Jump roping; it's takiing me a while to develop the coordination but that makes it fun. I find it is strengthening a lot of little muscles in the arches as well as the big muscles in the calves. I practice indoors on a yoga mat or on the soft/rubber floors at the Gym.
For tennis training, I'd also suggest high intensity interval training. You can do this w/ weights and kettlebells; you can also do this w/ body weight exercises and sprints. This mimics more closely those points where you run back and forth on the court and then have to recover and start the next point.