A Macquarium would be my first choice.

Then, like mxlews said, there is the 'ol case switch. Because the Mini is so small, it would be ideal. Several things you could do with this. An obvious one is to get rid of the CRT. Conversion wouldn't be worth it and a 14" LCD would be much nicer looking. Check eBay for cheap options. Then you could paint the case and add your touches to it.

Anything else would probably not be worth the expense.

Don't forget to submit a guide when/if you do it!