easy..
what to mine? BTC
how? well if you TRULY arent paying any electricity costs, then you should buy the cheapest old miners out there like some BFL or Avalon G1 old scrap.. this will maximize your Ghz output per $ invested.
after a quick look on the market, I am pretty sure there are many desperate sellers who want to get rid of their old miners for really dirt cheap.
a hash is a hash whether it comes from an S1, A1 coincraft, or a KnC miner..