8th grade is old enough to do just about anything. I like to do big memorable projects with my kid where we both have to learn some things along the way. I’d build a go kart at this age, or some other electric motor driven vehicle. I’d think it would span a few months depending on time dedicated. And covers mechanical, fabrication, components and even some programming if you want to tune it yourself.

Also, I like to reinforce the concept of “dreaming it up”, tell my kids to think of a thing to make and we break down the steps we think are needed to build it. At first, it’s like their imagination wasn’t big enough. They’d have an idea and I’d show them how we could build it pretty easily. Now, it’s gotten to where they know the skies the limit and anything of kind of possible so they start big and then work their way back down to a realistic scope of features. All to say, ask your kid what they want to build!

This is a great way to teach long term planning and goal setting.