Spend an evening filling your phone calendar with free activities
You'll find free or cheap activities on Eventbrite, library, museum, art galleries, theatres, summer fayres, heritage days, cinema, look for things like Science festivals too.
Then I'd make a routine for weekends without anything on. Make a good weather routine and poor weather routine, with items at hand for either. Having a routine gives the children structure which they like and takes the stress from you. My mum remembers her family's weekend routine fondly 60+ years later.
A typical weekend when DD was that age would be walk down the high street, visit the charity shops and DD would buy a new toy for a £ or two, pick up some lunch normally french stick and ham etc, ingredients for cake making later, library and check out books, walk home via the park. Get home and have lunch, bake cakes. Read library books, decorate cakes.
Sunday, beach walk, play at beach park get home and warmed up, movie, board games, generally a chill day
General activities Items I always had on hand for lulls would be things like
Decorate biscuits - Rich tea, icing sugar and icing pens (DD still loves this as a quick activity as a teen)
Ingredients for salt dough
Wet weather gear and flasks
Paints/pens and a big roll of lining paper