- Play "identify the waterfowl in the park" followed by "chase!" and (for larger geese and swans) "run away!". [This should satisfy the other comment "play sports instead".]
- Then, collect sticks in the woods and build a shelter. Extra points for using different types of structure, or working out which tree the different sticks come from.
- Collect different leaves and paint them. Or paint rocks.
- (In a year or two, you can get a plant press / heavy books and dry flowers and leaves between old newspaper.)
- Grow seedlings. Grow cress and eat it, or tomatoes, or just flowers.
- Search for fossils, bones, seeds (acorns, conkers, pine cones etc)
- Make a paper or balsawood boat, and float it on a pond
- Look for sea creatures in a rock pool
- Make an insect haven, then look at / identify the insects
When I was 3-4, I had a table placemat called something like "At The Seaside"[2], with typical creatures found on the British coast. My sister had "In the Garden". You might find something like that at a local museum, or some other child-appropriate guide to local flora and fauna. The museum probably has plenty of other STEM toys.
[1] https://www.nationaltrust.org.uk/features/50-things-to-do-be...