Thrift stores and butcher shops are a great place to look for
large rolls of blank art paper.
Kids LOVE to scribble and draw, usually on anything large
surface they can find (like walls) so if you can find some
large rolls of paper you will have a never ending supply for
them to draw and scribble to their hearts content.
Tacking it up on walls or ripping off a large piece to
cover a table will keep them occupied for hours!
Give them crayons, washable markers, finger paints, glue,
sparkles…anything you can think of that they would be
interested in using and you can be on your way doing what
you would like.