Here's an engineering challenge that will keep the kids engaged for quite a while: have them build a bridge that will hold a certain amount of weight.

You can use any common household item you want to do the testing, or a toy car if it's heavy enough. You could even make it a competition to see whose bridge can hold the most weight! The possibilities for building materials are endless - craft sticks if you have them, or wooden skewers or, better yet, the contents of your recycle bin. It's a good idea to get a sense of just how bridges work to redistribute the weight before beginning.This site has a good diagram of different types of bridges. If you don't want the mess of construction with recyclables etc., Lego is always a good choice as well. 

If the first bridge doesn't hold up under the weight...back to the drawing board! Just have fun with it, and your kids will be picking up some physics and engineering basics without even knowing it.