Buy a luxury yacht.  Obviously!  Under IRS rules, a boat may be considered a second home and therefore elegible to use the mortgage interest tax deduction.

Tom Kelly at Inman News writes:

According to the Internal Revenue Service, a second home can be a boat as long as it has sleeping, cooking and bathroom facilities. You can also deduct the mortgage interest on a boat as long as it is used as security for the loan.

Hmmm....that makes my decision easy.  Now if I could just find $100 million sitting around!