Flat-Pack Wooden Mallet
This flat-pack mallet from Vert Design may not be as effective as a traditional wooden mallet, but it looks like it'd be fun to assemble. Too bad it's forty bucks! (They'd have to hit me in the face with it a few times before I dropped that kind of cash on a less-than-solid kitchen gadget more clever than practical.)
Product Page [VertDesign.com via Gizmodo via MoCo Loco]

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Now THAT you could smuggle on an airplane.
It's $40 Australian, though, which was once about half $40 US. Admittedly I speak as a member of the designer's family, but it does work very well as a toy, both in the making and in the use.