First, consider the age of your child. Is his attention span long enough for a week long project or will a few hours and then success be better spent quality time? If you are lucky enough to have multiple generations in your household (or even a grandparents visit), make sure to include grandpa. I did a bit of research on the internet and found some of the best answers that can help you on your chosen project.
On Wikihow we found:
Make a soapbox derby car
Build a tree house
Make a paper boat
It looks like WikiHow really wins on their in depth explanations on building!