Buy something from Amazon that you were going to buy anyway. But just use the mall to get there. Simple. In previous years I too bought B&N gift cards as well as Sears gift cards. As long as it doesn't say "no miles earned on gift card purchases" you should be fine.
I used an Amazon gift card to buy something from Amazon through the DM Mall link and the 30 miles posted. But apparently YMMV based on Nikolaos' experience.