I don't live in the countryside but take my GCs out to just be in nature.
Walking is free, let them take photos, pick berries, throw twigs in the water etc.
Autumn is such a lovely season and if you live in the countryside you have a wealth of things to see with them; horses, sheep, cows etc.
You can get phone apps now to identify bird song and plants.
On rainy days baking is a great option but if they are busy at school during the week they may just like to chill out.
Talk to them about your own childhood, show them photos, take them to see family and /or friends- encourage their social interaction and conversation skills.
Tell stories, write stories, read stories- there really is no end to what you can do with a little imagination.