Yes! Another children's game we have gleefully co-opted for tennis! This one is from tennis professional David Brouwer, presented during a PTR Symposium. Best for players old enough to multi-task as well as rally (tossing or hitting).
A topic/category is selected (animals, places, cars, sports teams, foods, words beginning with 'M', etc.). After each hit or rally, player(s) must say a word from the topic/category previously agreed upon. When there is a miss, either with the tennis or with the word selection, next player is up. If you want to play this as an elimination activity, last player standing is the winner.