Tips for Very Simple Machine-Making for 6-Year-Olds?

My near-6-year-old is wild about building what he calls "machines" out of Legos, blocks, boxes, and everything else. I thought it would be fun if he could build some actual machines as well as the imaginary ones. Lego's Mindstorms robots look fun, but they are said to be for age 9 and above, and I'm not sure whether my boy (let's call him "Ben" for convenience) will have the attention span to enjoy them, even with my help. And while I'd be happy to do the programming, with him watching and (I hope) learning, I thought he might enjoy something that he could quickly do entirely by himself, at least as an option.

Any suggestions? Any experience you folks have had yourself with your 6- or 7-year-olds? Many thanks!