I remember in heatwaves, at that age when we'd spent the weeks's Big Day Out budget, I'd take them to the largest Tesco Extra or similar as it was beautifully cool. Ice creams and I'd get them to choose some picnic stuff..
A trip to the swimming pool always cheered them up and wore them out.
Lunch outside as its always better in the open air. Bikes/trikes round the park.
A cheap paddingling pool, A tent - could even fill with those ball pen balls.
Water gun fights.
There was an appliance shop near us that always had big cardboard boxes, which they love to play in at that age.
Also, if raining (unlikely) but also its probably air conditioned.. if there are any garden centres that have aquariums.. that was always popular.
And by that time they were ready to veg out in front of a children's program, which gave me a chance to breath for a bit.
Audio books if driving.
Extra long bath times with Audio books and toys
They didn't have the patience to do baking or cooking at that age unfortunately, but if you could get them decorating biscuits or similar
Chalk drawings on a paved area.
I'm sure there will be some other suggestions.