A 6 or 7 year old has no intrinsic interest in making mileage. His interest will be in whatever is surrounding him from moment to moment that he can engage with. The ideal backpack would be an overnighter, fairly short hike, with lots of cool stuff to explore, like a lake shore or creek side, places to play in rocks or trees, and a campfire, which right there is hugely fascinating. The point about not many biting insects is important, too.

If you are 1:1 the whole time and keep your eye on him, not letting him wander out of view or earshot, he should be safe enough. But do not rely on him understanding how to keep himself safe, even if the rules for what NOT to do seem obvious to you and even if you've told him ten times already.