If you have a garden I would invest in some decent toys/equipment to keep them amused. At those ages my daughters favourite things were potion making/mud kitchen, trampoline, water table and sand pit. Plus ride on toys, water pistols, cheap solid plastic pool (we did one each!) We hunted car boots/charity shop for cheap figures, old tupperware and plastic houses to play with in the water.
You can get gazebos fairly cheap so can use garden in all weather. (aldi do a fold up one, I regularly put these up and down alone at work).
Facebook marketplace you can get some great bargains, the money you would spend on 1 day out could provide a whole summer of stress free fun!
I work in a preschool we use builders trays (or sometimes called tuff trays) for a whole host of messiness. Loads of ideas online/pinterest. Or alternatively cat litter trays if you wanted to keep it individual and on a smaller scale. We do that now indoors all year round (I save free newspapers to cover the floor with!)
Or try and brave going out, my dh worked alot of hours when my girls were young so i did beach days, arcade (they loved 2p machines!), Saturday morning cinema or just park days on my own. We used to plan 1-2 days out a week, but went for a walk most days to burn off energy!! The more you do it, the easier it becomes, I dont drive either so we had to get a bus/train to most days out!