Mine are older teenagers now, so it's been a while! But, I do clearly remember those long days of needing to fill time.
Soft play was a waste of money for mine - they just didn't seem to enjoy it, and spent the time whinging to go home 🙄.
From memory, the free/cheap stuff that seemed to be a hit with mine..
"Washing up" - a basin of warm soapy water, and a selection of plastic cups, plates etc. Give them a sponge and a teatowel.
A tip I got from MN about 15 years ago was to let them 'paint' the garden fence. Get a paintbrush, and a tin of water, and let them get busy. A wooden fence will "change colour" when it's wet.
About once a week, we used to go on a bus run. The local bus would stop by us at around 10.30am. We'd pack our lunch and drinks, get on the bus and just ask the driver for a round robin ticket (I think it was £3). We'd tour around the local villages, looking out the window spotting things. The bus would drop us back home about 1pm, just in time for nap time/quiet time.
Possibly the best money I ever spent was £50 annual pass to a local garden centre, and we renewed this for a few years because we got such good value from it.
Aside from being a garden centre, it had a dinosaur display, a little museum of space travel. There was a miniature train to ride on, a small soft play area, a swing park, and a cafe. Everything included in the price (except the cafe obviously).
I know it's quite a bit of outlay, but we went at least a couple of times a week. The DCs would wake up in the morning and immediately ask if they could go and see the dinosaurs - the stegosaurus had a button to press which made its head turn and make a roaring sound. They never seemed to tire of this!
I'll think of some more.
It's a really hard stage, Op, I remember it well.