Vary the dog toys regularly, this way there is less chance of boredom. Make sure you have different kinds of toys, one to walk around with, one with treats, one to play "soccer" with. Hide toys, a lost and found toy is so much more fun than the one that is always there. Let your dog search for his toys, that is a fun game on its own. Buy toys that are easy to clean. There even are toys you can put in the washing machine or dishwasher.
