Tough question. I started my kids at birth and I enjoy seeing them grow through the learning. Their interest and questions change as they get older. They go from simply wanting to touch and taste it all to wanting to know WHY and HOW and things I don't even think about.
With kids that aren't mine, that just come to my house, they are interested in the animals and that's it. No interest in the plants. These kids range in age from 5-teen. They are also devastated when I mention we are most assuredly going to eat the animal they are obsessed with.
The school has my kids on a recycling kick though. It's put me in a hard place as we don't actually have recycling where we live. They even shut it down in the closest city.
Great question, Dave! I think my favorite age group is those aged 3-5. They are so interested in everything!
Of course, that age group can be exhausting, too! My favorite thing to do with them wasn't so much a "project" as a guided activity. We'd go for "nature walks"!
Whenever the kids got squirrely and the playground was closed, I'd take them for a walk around the building. There's nature to be found even in a walk around the building. I'd ramble about plants, pointing to a hemlock and telling them it's name, and the point at a cedar and tell them it's name. Then as we'd walk, I'd have them tell me if a tree was a hemlock or a cedar. I'd take a bit of the branch off of both trees and show them how they looked different and let them each touch it. We'd ID other plants, talking about whether or not they were poisonous. I'd name birds we see, and talk about clouds. Really, I just blabbed on about whatever I saw and asked them questions about it "what color is this flower?" "how many pansy flowers are there?" etc.
These walks were great because it got their wiggles out, and it gave us teachers a break from kids fighting over toys, etc. There's generally very little behavior problems when you're walking outside, and they don't yell and fight because they're listening to you talk or looking around.
Now I no longer teach, but I do have my kids at home, and I still take them for walks. They usually beg for them, and we'll walk sometimes for miles. When they were little, I'd push them in wheelbarrow or pull them in a wagon, or wear them on my back. When they were babies, I'd just ramble about plants and colors. Now that they're older, I talk about the various plants that grow on cedar stumps and why they grow there, why the trees died in the wetlands and how they provide habitat, how beaver dams work to clean our water, and point out animals, and answer their questions. Sometimes we'll sing songs, or I'll read a book. Sometimes we'll snack on dandelions or salal or other wild edibles.
I love the walks because it's such a sanity-saver for me--the kids aren't fighting! And they learn and get exercise and take better naps, and if we're walking down our gravel road we meet neighbors and chat.