I love the summer holidays. I work PT from home but scale it back so I have most daytimes free all summer.
I trawl websites for local museums, parks, councils, wildlife trust, plus National Trust and English Heritage as we are members, for free and cheap activities. There are loads. Last summer I spent about £60 all summer -and we didn't use the memberships much. I put all the activities onto a spreadsheet and then have a coffee with my friends and decide what we're going to do. I also have a list of free outdoor and indoor places to visit so we fill empty days with ideas from that list.
We found a family beach scavenger hunt for £3, a puppet show (£1 per child, adult free), free science activities at the museum, bug hunt for £1 per child). We did something every day.
I also keep a list of things we can do at home. They are on little slips if paper in a box and I get the children to close their eyes and pick one. Things like make a den, a board game, get a DVD from the library, bake a cake are in the box. Somehow not knowing what's coming out of the box makes it more fun.
If you invest an hour with google I'm sure you can occupy children all holiday.