We have toddler grandchildren. They love toys, and it doesn't matter if they're store-bought or not. Look up a toy on the Internet, then reproduce it at home on your 3D printer. It doesn't need to come out perfect, the kids won't mind.
A companion piece would be a plastic recycler. There are plans available on the Internet, detailing how to make your own machine to melt used plastic and reform it to a spool of plastic wire for use on your 3D printer.