You can use it as a DNS that will act as an adblock for all your subnet machine.
Checkout the adsuck project: http://freecode.com/projects/adsuck.

You could also use it a honeypot, or capture the flag box, or as a pentest test box.
Checkout the mutillidae project: http://sourceforge.net/projects/mutillidae/files/.

What about using it as a little retrogaming box that you connect to your TV.

Just be a little bit creative.