My dad is a builder. There was always wood around when I was a child and I loved that. Great for make-shift cubby houses and other fun in the backyard.
At places like Bunnings, you can buy cut pieces of wood for craft projects (starting from about $5). A few pieces of wood in the back yard has proven to be great fun for my kids. They can set up their own obstacle courses, play games (like Pirates!) or simply do balance games. Just a simple idea.
Idea: paint or decorate the wood with paint or permanent marker for fun.