If there are woods near you, that's half the holidays sorted for free. DC used to: make dens, climb trees, balance on fallen trees, hunt for mushrooms, make mudslides into the stream, build dams in the stream, ride bikes, build mud ramps for bikes, make rope swings. And my two are very far from sporty boys. They just loved playing in the woods.
A bit of heavily supervised gardening or DIY project. DC 'built' IKEA shelving units and painted them with me; sawed logs, built and tended bonfires, grew veg.
Big box decoration. We always kept boxes that white goods came in. Then in holidays they'd become house/rocket/car etc.
A trip to Poundland. Give them £1 each and set them loose on the craft section of Poundland or Wilkos, then come home and use whatever they chose.
Film night: home made popcorn and home made pizza slices in a darkened room with a family movie on.