Until this year we've always gone the same places that have lots of nature and geomorphology. You could do courses on foraging, that was really interesting for when they're older, survival in the wild type thing. There's so much out there we can eat that we don't.
If you have a wood burning stove or even for at home, foraging in a woodland for wood is exciting, they'll need gloves. Foraging on beaches for shells, rocks & wood. On the East Coast there's an area with a ton of fossils, this Dad was saying how his son is fossil mad so they go there every now and then living near London.
Depending on your beliefs the Coast has places where you can go out to sea on a tour and to fish. Or if you don't have sea legs, beachside locations offer you a cook the crab you caught service.
I got books on tree names, bird names, plant names. They had a book on outdoor wildlife based exercises for children. Pitching up a tent and getting back to nature. You've got the Lake District & Peak District plus areas all over that will offer an array on camping services. From your really basic to really plush with glamping where your night under canvas is quite luxurious.
My DC share a microscope & catch things to look under slides, from say soil to leaf patterns to samples of water from ponds. Dove Dale is quite fun on a good day as you've got the stepping stones, there's a few fossilised rocks, the water is so pure but also freezing. If you're all feeling fit enough you can climb the hill which will give you a view for miles. In the same area you've got caverns you can travel to.
It's thinking about what you want to achieve, also considering safety. I don't know about others but I swam in rivers as a child, you'd never do that these days. So waterside safety, water safety thinking about what discharges into some water courses and beaches.
Someone tried to tell me ages ago that the Thames isn't actually mucky, it's just the silt from the tidal system. Not sure how true that is. Last time I looked I wouldn't 'filter the sediment' to clean it.