The library do fab free arty days and their free reading quest thing is always good(think it's space this year).
We got the inlaws to get us a family NT card which has been fantastic. They do Wild Wednesdays throughout the hols with themed activities or we'll just explore the gardens.
We'll be going to the beach,woods and local farms a lot.
The website Kidscraft weekly is fab for cheap craft ideas with stuff from around the house. The Crafty Crow site is also good.
I'm also going to get out all the old board games and sort though the toy cupboard getting the dc to play with all their Lego/Playmobil they never have time to normally.
I also google around and make a list of cheap things to do at home if they get bored eg dd wants to make a fairy garden for her plastic fairies,I'll get the boys to do the same with dinosaurs.
I've got them some new paints and have been collecting junk and cheap cellotape,glue etc. Might make a holiday scrapbook.
I also go through the Primary Times and make a list of all cheap/free stuff,plotting it on the calendar.We then do the free activities and have our chill out days on days there is nothing free on.
You don't need to spend a lot to keep them amused but I do think it takes a bit of forward planning. Having a few cheap ideas up your sleeve stops you panic spending iykwim.