Walking (even in winter) once I get my back and shoulders sorted - but I would have that day to go to physio, swim etc and improve things drastically!! Then get back to my beloved hiking.
Craft things - I have big plastic bins full of fabric, wool, cross stitch etc and some paper craft materials, with loads of projects in my head just no time to do them anymore. With a day per week I could set aside even just 1 a month to crafts and start to make progress.
Actual reading - curled up to go through some of the shelves of books I bought wanting to read and never getting time to. I would probably also take a book to a coffee shop for a couple of hours, and then wander around the galleries and museums in town that I love but never get to as lunches are taken at my desk now (when I am in the office) to get things done. Or cinema matinees. Or time for beauty/hair appointments that are relaxing as well as maintenance ones.
Yes, I would also do some housework (especially deep cleaning or maintenance jobs) and grocery shopping and batch cooking some of the days, to get me caught up and make life less stressful in general. Which in itself might give back some time on other days to read/craft/relax in the evenings. I would probably try to organise myself to have a big kitchen day (including early shopping, batch cooking and some baking) once a month, filling freezer with decent dinners (rather than the current "fry chicken, open jar and toss in sauce, add frozen peas if I remember for veg, boil rice/pasta, serve" dinners that are happening far too often).
Proper gardening again - I used to grow a lot of veg (I even had an allotment for 5 years and grew most things from seed), but do hardly any nowadays, just a hanging basket of tomatoes (from already grown plants from GC) and a couple of pots of salad leaves.
Oh, I would have plenty to fill my days, even in the depths of winter.