My son is six years old and has always been obsessed with how things works. He loves to figure things out and he's pretty amazing at doing it. Any time I work with my tools he likes to be right there next to me. And whenever one of the toys in the house runs out of batteries, he is the first one to grab a screwdriver and batteries and replace them. (Thank goodness because I really don't like doing it!) I've never bought him play tools because he doesn't want to PLAY with tools, he wants to USE tools. So for Christmas I bought him his very own set of real tools! I found a small plastic tool box and filled it with a small hammer, screwdrivers, a tape measure, a wrench, pliers, oh, and some batteries so he can replace all the old ones to his heart's content. I also got him a small battery operated screwdriver.
Then, knowing his love for using a screwdriver, I made something for him to practice on. I got a wood plaque and the letters to his name, painted it, and then pre-drilled the letters to the plaque. I then outlined the letters with a sharpie so that when he removed them, he could put them back and match up the holes.
He loves his tools! And he has something he can really put together and take apart with them.