If it's a rainy afternoon, or if the kids are just being unbearable little monsters, here's a tip for you. You can print out free coloring pages from the Crayola website.
Apparently there are over 1,000 pages to choose from!
Some of the designs seem quite technical, such as those in the Cars, Trucks, and Other Vehicles category. You can design spaceships, gondolas, monster trucks and more.
So, if you have a laser printer with relatively cheap printing, you could print out a lot of these and store them in a file for future use. If you have an inkjet printer, printing is a bit more expensive, so maybe only print out the pages when you need them.