When you take your dog for a walk, let him sniff.
Smell is the most important sense for dogs. Dogs navigate the world with their noses much like humans use their eyes.
Use this to your advantage when you take your dog for a walk. Let him really discover and explore his world by sniffing. He will get so much out of this, sniffing will make his walk much more enjoyable and will also help to tire him out.
Sniffing is an easy to do, mental enrichment exercise for dogs.
Have fun with your dog!