These are great little projects for any little boy.
They come with everything you will need (accept for the hammer) and in a variety of kits. They have everything from a birdhouse (which is what we did) to coin banks.
For under $8 these would even make great gifts.
The instructions are easy to follow and can be built, with adult supervision, buy a child from 5-10.
The smaller nails can be a bit tricky for the younger kids and unless they are hit in just right can separate parts of the wood.