my kids love to pretend to do cooking- get the pots and pans out of the cupboard and they pretend to cook. Fill up the sink with water and let him wash all the plastic dishes.
Build a cubby out of sheets and blankets.
Make fairy bread, cut out with star cutters
Playdough- so easy to make, the best recipe is on the cream of tatare box
My kids love to 'clean' so I usually spray my back sliding door and kitchen cupboards with wather and let them clean them down with a sponge
Make and fly paper aeroplanes

Good luck- we are a bit the same, on about day 5 of a heat wave in Perth, and everybody is soooooooo over it.