OK you have a computer and a camera (I hope, everyone has a camera on their phone now don't they?).
I think the kids need to make a newspaper of what they have done this week with lots of pictures.
So yes you might get wet in the park but you will also get some great pics of the kids splashing in puddles or feeding squirrels.
When did you last go to a place of worship? Most churches, temples, mosques are happy to have visitors and will show you round and often give you a cup of tea and some will even feed you. Cheap and educational.
Cook something or bake something, you have to eat so make it in to an activity.
Any spare pasta can be used (uncooked) to make pictures.
Garden centres (if you can get to one without a car) can be fun lots have pets or you could buy some cress to grow on cotton wool.
Pet shops are free to look round.
Borrow a dog and take it for a walk.
Don't just mooch around the woods, make it a nature trail, can you see any wildlife? Can you see any signs of wildlife such as tracks, what are the tracks for.
And if you take some string / washing line you can build a den.
List which trees would be best to climb and why (or maybe you don't want to give them ideas).