When the kids were little, I used to keep two very important boxes on a shelf over my my washer and dryer. One of these boxes was a Craft box and one was an Inventions box.
In the craft box I would toss bits of lace or ribbon, found markers,crayons, big buttons, used greeting cards, tubes of glitter I had bought and any found or cheaply purchased items that I thought would be good additions to the box.
In the inventions box I would place broken radios and clocks, juice can lids (the kind that come from frozen concentrate with tear strip - no rough edges) soda pop caps, any odd ball pieces that I thought would be good invention material.
On rainy, cold, or otherwise bad weather days, I would get down one or both of the boxes and let my son and daughter spend the day just exploring the boxes and making whatever they wanted from the boxes. It was always a big hit and made even better because they never knew what would be in the boxes because I had been adding to the boxes since they last looked in them.