BBC things to do website has lots listed by area - most are free or v low cost:- www.bbc.co.uk/thingstodo
Making a film - digital camera with video function or mobile phone. Set a theme or let them choose. Or shadow puppet show (cut out pictures stuck on kebab skewers).
Paper crafts - print out 3D frozen dolls from Disney website or 3D shark from twinkl etc. etc. Loads out there for free - just ink, paper, scissors and glue/tape.
Nature treasure hunt:- www.naturedetectives.org.uk/packs/treasure.htm
ispot - www.ispotnature.org/communities/uk-and-ireland - They had a poster to print out last year with things to find every month.
We are going to go on a mission to get all the blue peter badges (except the one where you have to save 100 pensioners from a burning building then deliver triplets in a bus station toilet)
Picnics (I mean, you are probably going to feed them anyway so may as well drag it out and make an event of it).
Paper/cardboard boat contest (in local river or a paddling pool). Best boat, fastest boat to get across/quickest to sink, biggest boat etc. etc.
Waitrose cafe. Free tea/coffee for you (with waitrose card), then £1 for ice cream per child.
Library - summer reading challenge and often there are free crafts.
Do you have Love Parks Week locally? Usually first or second week of the holidays - some councils run free events.
National Trust open spaces (like commons or heaths). Usually free entry with £2 for parking. Often they have free events on too, like den building day or treasure hunts.
Swap children with a friend - they have them in the morning, you have them in the afternoon.
We used to go to Gatwick Airport on the train for the day (with packed lunch) and ride on the shuttle between terminals. Do you have anything similar nearby? Not an attraction you would immediately think of, but an interesting place to go for children? Somewhere functional, free to enter and with public toilets!
Have a pile of printed out worksheets and when someone complains they are bored, produce hard sums. Amazing how often children will find something to amuse themselves when faced with homework or housework.