Free/low cost (depending on what you already have in the house and budget) things my children enjoyed doing during the holidays when they were little included
Junk modelling - collect recycling stuff and give them some glue.
Baking.
Going out to collect stones and painting them. We used to take them back and hide them in the park- not sure if that's still a thing!
Painting cheap terracotta plan pots to make fairy houses or whatever (tubes of inexpsnive acrylic paint can be bought from the works. I used tester pots of emulsion because it was cheaper).
Tearing up strips of fabric (eg old clothes or bedlinen) and tying to string/garden twine to make rag bunting to decorate the garden/bedroom.
Campfires in the back garden - especially for toasting marshmallows and sausages for hot dogs.
Using a small cardboard box and collecting things from the garden to make a bug house.
My daughter and I once bought a cheap mirror and decoupaged the frame (we used pva glue not specialist stuff and I still have the mirror hanging in the house). You can use specialist decoupage paper, old comics, tissue paper, wrapping paper - any thin paper really.
Making dens in the house and garden.
Camping out in the living room overnight watching films.
YouTube karaoke - they're adults and we still do this now.
Picnic in the hoise/garden - put a rug on the floor and eat lunch.
The National Trust free days.
Who can make the best paper aeroplane competitions.
Making things out of salt dough or painting and creating a gallery.
Leaf printing/nature rubbings.
Who can crawl from one end of the living room to the other fastest in a sleeping bag competitions.
I taught them how to go downstairs in a sleeping bag 🤷🏻♀️
Creating indoor/garden obstacle courses.
YouTube drawing and origami tutorials.
Gardening, planting stuff.
And even if you have to buy stuff, you can use them all more than once.
I was a single parent on a low income at the time and, tbh, I'd still choose to do all of those things in favour of an expensive day out somewhere! I'd forgo expensive ice creams for any of them, tbh.
Ah, its been lovely remembering! Bring on the grandchildren 😉
ETA: my point is that there are plenty of free things you can do for the rest of this week and plenty of low cost things you can begin to prepare for for the summer holidays.
It doesn't need to he expensive.