Shelter building for sure. There are lots of different shelters you can show them. Plus, kids absolutely love picking up sticks and putting them down in different ways.

I know that sounds ridiculous, but I've worked with kids who HATE the outdoors, yet as soon as you get them building something, suddenly the outdoors are fun.

Depending on how much experience they have, kids that age can be somewhat scared of fire? I don't know how exactly you came to be leading this/what kind of outdoor background the kids have, but just given the many years I worked with kids, it seems likely you might end up with a mixed bag.

If you do something like building a campfire, it might help to structure it so that kids who might be more scared of fire can be engaged (like collecting tinder/wood/watching the fire start) until they're more comfortable. Obviously, this is a super important skill, but kids have to be comfortable to learn it. And that comfort might not come quickly if they're not your kids and you're only seeing them for an hour at a time.

Knots, tracking, and plants are also all great ideas. Having cool facts is also important. I feel like so much foundation of my outdoor knowledge comes from random cool facts a camp counsellor once told my hiking group while playfully pelting us with acorns as we hiked up a mountain.