On the Crayola colouring website, there are literally THOUSANDS of free printable sheets for colouring-in! These are great for your kids to waste away a rainy afternoon.
There are various categories of colouring activities, such as holidays, or Disney. If there is something specific you are looking for, you can even use the search box to find it. They pages are pretty varied, with everything from Albert Einstein to The Queen!
As I child, I used to love colouring-in. It was one of my favourite pastimes. I remember getting those Crayola art sets for Christmas, and spending weeks drawing and colouring. Good times!