Building dens in the living room, with all the sofa cushions, big sheets, blankets and cushions inside to make a cozy place to curl up and read.
Lots of trips to the library, DS will read a book in an hour if he gets the chance, so regular trips to the library for more reading material is a must in the holidays.
Torch lit walks through the local wildlife reserve once it gets dark, then home to hot chocolate.
A trip out of the house at least once a day, even if it's just a brisk walk around the block or to the pub for hot chocolate and a cappuccino.
Baking and generally helping with the meals, cooking a meal from scratch for the whole family. This can take up lots of time if you include pouring over cooking books, planning the menu, shopping, writing up the menu like in a restaurant, laying the table etc. etc.
Lots of random woodwork and creativity using scrap wood we collect, the drill (under supervision) screws, hammer, nails, wood glue and dowels.
One winter we made a massive marble run using cardboard tubes, pipes, bottles, and other junk, we spent weeks designing and constructing it, and we attached it to the staircase wall so the marble run started on the first floor landing and finished in the sitting room downstairs. We were due to redecorate our walls in the spring so didn't mind sticking all the pieces to the wall in the meantime. I think it was the best thing we ever did in terms of value we got out of it. Who doesn't like a marble run?
Similar to that is car tracks and car ramps, using all sorts of things.
craft and sewing and creating are also things DS does a lot, along with painting and drawing and designing his own comic books.
And when all else fails, we watch films, play on the computer and play on the playstation.