I have a trained spaniel, when the trainers got him he had zero interest in toys, but they needed him to want a ball so that he could be trained.
So they made him think that a tennis ball was the best thing ever. They removed all toys from him other than when being trained, then everyone in the room/area would act super over excited playing with the ball. basically four adults played catch, chased it as it rolled across the floor and shouted excitedly when we got to pick it up. He thought we were crazy to start with, but then tried hard to get at this great thing. If he beat us too the ball, lots of excited praise. Took a few half hour stints over two or three days and he's totally hooked for life, will do anything for a ball.