Puzze toys are a great way to make your dog use his or her mind. Getting them to think will tire them out as much as taking them for a long walk, and a tired dog is a good dog! You can Google "Dog Puzzle Toys" and all kinds of different toys will come up. In all cases, they'll have a place where you put treats or kibble and the dog will have to manipulate something to get at the reward. Different types of puzzle toys appeal to different dogs, so you may have to experiment a little to find ones that really motivate your dog.
If you're at all handy, check out the video link below for a great "do it yourself" puzzle toy that will keep your dog entertained for hours.