The town I live in - Milton Keynes - has an amazing Play Association which runs lots of regular sessions throughout the holidays in parks throughout the town, so I try to go to at least one a week as something to aim for to get out of the house.
They also run a scrap store which sells amazing bits and bobs very cheaply (companies give them things they don't need that they'd otherwise need to pay to have taken away, scrap store sells them cheaply) - so for example, a company that needs to cut plate size holes in wood for their product give them the round cut out bits, another company that used yarn on big cones that needed to be thrown away gave the cones - which were fab for all sorts of Christmas tree craft projects - that you can then buy really cheaply and they're great for craft projects.
So a long holiday will always involve a trip to the scrap store to get some bits and bobs for craft projects - for a fiver I can get a bootful of bounty that lasts the dc for weeks. They have a whale of a time planning what to do, they love looking at the store itself and of course they then spend hours doing their crafts.
(there are lots of scrap stores around the country, they're a fantastic resource and I wish more people knew about them - both to use them and to be able to donate to them)
DC are just getting into geocaching - which can provide a 'reason' for going for a walk or exploring somewhere a bit different. And now of course there's Pokemon Go which is a very similar thing, just lots cooler when you're 11
. Our council has also started to make apps to encourage people to go out and do things - like treasure hunts for example - to give other things to do.
Benign neglect is also important - making sure the tv and all devices are off and so they have to do something else if they don't want to be bored - but it's important to have things around for them to pick up and do - craft stuff, books, music, games, balls, racquets, lego, basic cookery ingredients and so on - so that there are lots of things available to do should they want to.
I'm always amazed by how much stuff is on locally - although much of it is very badly publicised - so it becomes a challenge to find cheap or free things to do. But it's also good to try and go to some of the stuff on offer - because if you don't and nobody else does, then they'll just stop bothering to offer it.
Setting the dc a challenge for the holidays can also be fun - but something that lasts a while and involves lots of different things, so it could be a surreal treasure hunt that involves their imaginations (Tomorrow's newspaper for example), or drawing pictures/taking photos/etc of plants/things/cars/whatevers for each letter of the alphabet, creating a holiday blog (or scrapbook if you're going old school).