Swimming, for days you have another adult pair of hands. Have you an outdoor pool near you? Ours is fairly expensive but we go early, take a picnic and spend the day (take books and toys for playing in the shade while the little one/s nap) - they're zapped and ready to pass out by 5 
Games day in the park - hop scotch (chalks from the pound shop), chucking bean bags into a tub, lime and spoon, three legged, use whatever you have or can borrow. Picnic too. Spend the afternoon hunting for things for a nature collage and then an activity at home sticking it all to paper.
Bread making? Hugh FW has a recipe for 'magic dough' that is cheap and easy, older one/s can help then make shapes (letters, plaits) and eat them for lunch, littlest can just smoosh the dough.
Biscuit decorating- buy a pack of cheap digestives, make vile coloured icing, unleash the smarties and jelly diamonds 
'Painting' the fence and house outside - buckets of water and paintbrushes, while you sit on the rug and play with the littlie, amazing how much they love this ??
TV! Rent a classic Disney film and have a movie afternoon with treats.