Two words, PAPER AIRPLANES....Children love to build them almost as much as they love to fly them. I will say this, it will teach you patience and your house will most likely be cluttered with them, but it is well worth it. The scenario goes like this....My 5 year old and I build them together by reading directions and looking at pics out of a paper airplane kit we bought him for his birthday. The problem is as we build them my 2 year old destroys them. If they were in construction....the oldest would be an architect and the youngest heading up demolition :). Anyway, if you have never tried it then give it a whirl. It will keep everyone busy for hours of fun.
You can Google paper airplane building or go to your local hobby shop. There are loads of different ones to build...I remember building the same one over and over and over again while growing up and it never seemed to get boring...just imagine if we had 10 or 15 different ones to build :)