Im a mother to one, but he is only 15 months old, so still home with me all day.
When he sees his dad though, i have a list of things i enjoy doing whilst child free.
Exercise classes.
Going for a run (i usually opt for this as classes are quite expensive!)
Baking.
Cooking a more time consuming dinner (i rush cooking when LOs home as he is generally clinging to my legs!)
Cross stitch/reading/watching an episode of my chosen "series of the moment"/having a bath & all relaxing things i cant do with a toddler about!
Cleaning/food shopping/all the chores!
I cant speak for women who are home all day whilst their kids are at school. Im sure i would feel the same if i was without him everyday, at the moment its a novelty as its only one day a week.
Could you do some volunteer work? Or a little part time job that fits in with school runs? Or attend a part time college course? Even if its just a "hobby" course to get you out & about & meeting new people with similar interests.
Whatever you do with your quiet days though, DO NOT feel guilty. Enjoy making time for yourself, i expect half past 3 comes round quick, & then its all about the kids until their bedtime!
x