I will be watching here as well, for my grandkids consider summer not complete without extended time spent at our place (without parents thank you very much!). We live in a rural setting, so need to make all our own fun.
My DH makes longbows, and a lot of time is spent in archery.
We make quilts.
I make a big pot of play-doh. The kids break off blobs, add colouring and make things.
We had a home spa day with cucumbers on eyes, avocado face masks, manicures and pedicures.
Boating and swimming in the lake.
Fishing.
The kids love it when we have work to do - for instance last year we were building a rock wall and they had a ball sorting and washing rocks.
No wonder their visits are exhausting!