We appreciate the importance to you and your husband of regularly playing tennis together.
There are two main halachic questions involved. First, is hitting the ball to each other considered similar to throwing to each other, which is prohibited during niddah? Second, is playing a game like tennis an activity that may lead to excessive frivolity or even physical contact?
Our site’s rabbinic supervisor, Rav Yehuda Henkin, permits playing tennis during niddah because the ball isn’t caught, but hit away with a racket, and because the distance between the players reduces the risk of frivolity. Your stated intention for the game to be more about togetherness and sport than romance (at least during niddah) further supports this ruling in your case.
Please let us know if we can be of further assistance.