I’d like to be active with my wife and play sports such as tennis with her. Is for instance, the hitting of a tennis ball over a net allowed while in niddah?
The poskim say that a couple should not play tennis ping pong or badminton together while she is a niddah. This is for two reasons, first of all because the game involves “throwing” the ball from one to the other, and because these kind of games can lead to the couple getting affectionate. Therefore it should not be done. See sources.
R’ M. Feinstein zt”l as quoted by R’ S. Eider zt”l The Halachos of Niddah 2- ftnt 37, R’ Y. Kaminetsky zt”l in Emes L’yakov ftnt 80, R’ S. Z,. Auerbach zt”l ( Kitzurr Shulchan Aruch chap.2 -7, MIshneh Halachos 5-150, The Halchos of NIddah By R’ B. Forst shlit”a.