Do you have extra boxes lying around? We always have a few, either from Costco or Amazon, and my kids have no problem finding another use for them. Caroline (5), requires hardly any direction on what to use them for, but Charlie (2) on the other hand needs a little assistance. Just the other week, Caroline decided to make a box into a Skye Helicopter, and then a small one for a cat bed. We've made a car out of a box, car ramps, used them to make paintings...the possibilities are endless. I definitely think boxes are a perfect way to introduce open ended play. Below are some pictures of what we have done with boxes in the past.
Seriously, just give them a box and see what happens before you start getting out the glue, paint and other tools. Happy Playing!