Pond dipping.
get some nets and a bowl, and a camera. Find a murky pond/rock pool and see what's in there. Take photos of mystery creatures and look them up after. Make a little display of the pictures.
Leaf hunt
Collect leaves, in park or garden, make rubbings of them with wax crayons, or draw them, photograph them. if you use pale crayons you can paint over to make a wax resist effect. Again look up which tree they're from.
Creepy crawlies
Find some biggish rocks, look under them for interesting insects, worms and so on.
And so on with shells, flowers, leaves etc.
We do a lot of lakeside picnics (well we are in Lake District) with paddling and swimming.
Have other parents for lunch, they can bring food too, or arrange to meet other parents out and about.
Have you any relatives you can stay with for a change of scene?