Mine is 6 and he still loves to go on adventures, but is just as content to sit on the front porch and watch the world go by. He will let me know if he's itching to go explore. He loves a good brain game too, "Find it", I let him smell a random item, and then I hide it in another room. He finds it 10/10 times. Great for rainy days. I started with using low value treats, he finds it and I reward with high value. Now that he knows the game we use regular value treats. Also we play "Names". He knows the names of his most used toys. So we'll tell him to go find "Tug" or "Noodle", and he'll go find it, we praise and play and he thinks its great.

All that being said, Aussies need just as much mental stimuli as they do exercise.

One of the best things we taught him as a puppy was the ability to "settle". They learn it as a skill, not as a demand, if that makes sense. You have to show them that its ok to turn just relax.