I have jobs and kids so am not in this (envious) position. I also can't imagine my house and garden ever being tidy enough - are there no cupboards or drawers or garage stashes that need you to go though and sort into piles for charity/tip/give away?
If I truly had nothing else to do I would read, I would lie in the garden and read or curl up on the sofa and read, I would have a long bath and read!
I would spend time in my appearance, have a long soak, sort my nails, sort my brows, play with some make up etc
I'd sort my wardrobe, out together different outfits, bin and tights with holes, tops with stains and wring size bras etc
I'd sort my present cupboard and start a Christmas list so I knew what I needed to look out for and budget for big birthdays and Christmas for the family thosbyear
I'd wash my outside windows, I'd wash the car, I'd clean the oven, defrost the freezer, sort the shoes
I'd write letters to elderly relatives, I'd sort photos into albums (real or online) and order photos to fit frames
I honestly, truly, genuinely can't imagine being at a loss for things to do.