Get stuck in with them. Take them out and ask them to think of all the things it could be - a spaceship, a hideout, a batcave, a science lab etc.
Search for old stuff around the house that could be used in imagination play - an old keyboard, phone, calculator, radio, old atlas? What about their toys? Do they have things you don't mind being left outside - a telescope, binoculars?
Can you modify a disguised secret hiding spot in it somewhere?
Old containers and food dye and water for making potions and science experiments.
Their imaginations are endless, I'm sure they'll come up with loads of possibilities.
DSs 5 and 7 have a tree house but it doesn't get as much use as I'd thought. Does anyone have any good ideas for accessories etc? I though maybe a pulley and bucket set up to start but then I'm struggling.